kenv MFC: r335998, r336019, r336026, r336036, r336217, r336335, r336337,

kenv MFC: r335998, r336019, r336026, r336036, r336217, r336335, r336337,
r336415-r336416, r336419

As an aside- this has been slightly altered from the version in head to keep
the MD and config-static environments mutually exclusive by default.

This difference is a one-line change in init_static_kenv to setup the MD
environment if the config-static environment is empty or if
loader_env.disabled is explicitly set to 0.

r335998:
kern_environment: use any provided environments, evict hintmode/envmode

At the moment, hintmode and envmode are used to indicate whether static
hints or static env have been provided in the kernel config(5) and the
static versions are mutually exclusive with loader(8)-provided environment.
hintmode *can* be reconfigured later to pull from the dynamic environment,
thus taking advantage of the loader(8) or post-kmem environment setting.

This changeset fixes both problems at once to move us from a semi-confusing
state to a consistent state: if an environment file, hints file, or
loader(8) environment are provided, we use them in a well-known order of
precedence:

- loader(8) environment
- static environment
- static hints file

Once the dynamic environment is setup this becomes a moot point. The
loader(8) and static environments are merged (respecting the above order of
precedence), and the static hints are merged in on an as-needed basis after
the dynamic environment has been setup.

Hints lookup are changed to respect all of the above. Before the dynamic
environment is setup, lookups use the above-mentioned order and fallback to
the next environment if a matching hint is not found. Once the dynamic
environment is setup, that is used on its own since it captures all of the
above information plus any dynamic kenv settings that came up later in boot.

The following tangentially related changes were made to res_find:

- A hintp cookie is now passed in so that related searches continue using
  the chain of environments (or dynamic environment) without relying on
  global state
- All three environments will be searched if they actually have valid hints
  to use, rather than just choosing the first environment that actually had
  a hint and rolling with that only

The hintmode sysctl has been ripped out. static_{env,hints}.disabled are
still honored and will disable their respective environments from being used
for hint lookups and from being merged into the dynamic environment, as
expected.

r336019:
config(8): De-dupe hint/env vars within a single file

r335653 flipped the order in which hints/env files are concatenated to match
the order in which vars are processed by the kernel. This is the other
hammer to drop.

Use nv(9) to de-dupe entries within a single `hint` or `env` file, using the
latest value specified for a key. This leaves some duplicates if a variable
is specified in multiple hint/env files or via `envvar` in a kernel config,
but the reversed order of concatenation (from r335653) makes this a
non-issue as the latest-specified version will be seen first.

This change also silently rewrote hint bits to use the same sanitization
process that ian@ wrote for r335642. To the kernel, hints and env vars are
basically the same thing through early boot, then get merged into the
dynamic environment once kmem becomes available and the dynamic environment
is created. They should be subjected to the same restrictions.

libnv has been added to -legacy for the time being to support the build of
config(8) with the new cnvlist API.

r336026:
config(8): Fix broken ABI

r336019 introduced ${SRCTOP}/sys to the include paths in order to pull in a
new sys/{c,}nv.h. This is wrong, because the build tree's ABI isn't
guaranteed to match what's running on the host system.

Fix instead by removing -I${SRCTOP}/sys and installing the libnv headers
with `make -C lib/libnv includes`... this may or may not get re-worked in
the future so that a userland lib isn't installing includes from sys/.

r336036:
kern_environment: Fix SYSINIT ordering

The dynamic environment was being initialized at SI_SUB_KMEM, SI_ORDER_ANY.
I added the hint-merging at SI_SUB_KMEM, SI_ORDER_ANY as well in r335998 -
this can only work by coincidence.

Re-do both to operate at SI_SUB_KMEM + 1, SI_ORDER_FIRST and SI_ORDER_SECOND
respectively to be safe. It's sufficiently obfuscated away as to when in
SU_SUB_KMEM malloc will be available, and the dynamic environment cannot be
relied upon there anyways since it's initialized at SI_ORDER_ANY.

r336217:
kern_environment: Give the static environment a chance to disable MD env

This variable has been given the name "loader_env.disabled" as it's the
primary way most people will have an MD environment. This restores the
previously-default behavior of ignoring the loader(8) environment, which may
be useful for vendor distributions or other scenarios where inheriting the
loader environment may be considered a security issue or potentially
breaking of a more locked-down environment.

As the change to config(5) indicates, disabling the loader environment
should not be a choice made lightly since it may provide ACPI hints and
other useful things that the system can rely on to boot.

An UPDATING entry has been added to mention an upgrade path for those that
may have relied on the previous behavior.

r336335 by arichardson:
No longer install sys/nv.h and sys/cnv.h in lib/libnv/Makefile

Use tools/build/Makefile to install the headers into ${WORLDTMP}/legacy
instead. Compared to r336026 this has the minor advantage that it avoids
unncessary header installation when building the non-bootstrap libnv.

r336337:
Unconditionally build libnv in legacy

Rather than using a config(8) built from new tree linking libnv built on
host.

r336415:
config(8): Add compatibility shims for r335998

Plumb the %VERSREQ from Makefile.<arch> through to the rest of config(8).
We've recorded the config(8) version that we're calling "the end of
envmode and hintmode," and we'll write them out for earlier versions. Later
kernel version bumps will remove envmode/hintmode from the kernel as needed,
which is OK since the current kernel does not use them at all.

These compatibility shims really need to go away when the major version
rolls over...

r336416:
Fix GCC 4.2 build after r336415, proper declaration and prototype

r336419:
config(8): Invert checks; envmode/hintmode reflect "env provided"

Relnotes:	yes (maybe) [The loader environment may now be used with
the config-static environment by specifying loader_env.disabled=0 in the
config-static environment]