MFC r337321, r337435, r337707, r337740, r337834, r337836, r337968

MFC r337321, r337435, r337707, r337740, r337834, r337836, r337968

r337321:
Make it possible for init to execute any executable, not just sh(1)
scripts. This means one should be able to eg rewrite their /etc/rc
in Python.

r337435:
Move description of init_shell, init_script, and init_chroot kenv
tunables from loader(8) to init(8), since it's init that actually
uses them.  Add .Xrs at their old place.

r337707:
Move around text in loader(8), in particular stuff related to ZFS,
to restore the usual section order.

r337740:
Add init_exec kenv(1) variable, to make init(8) execute a file
after opening the console, replacing init as PID 1.

From the user point of view, it makes it possible to run eg the
shell as PID 1, using 'set init_exec=/bin/sh' at the loader(8)
prompt.

r337834:
Add SECURITY section to loader(8).

r337836:
Improve formatting.

r337968:
Consistently use NULL to terminate the argv; no functional changes.

Relnotes:	yes	(init_exec kenv(1) variable)