MFC r227827

MFC r227827
  Define __FreeBSD_kernel__ macro in sys/param.h.

  __FreeBSD_kernel__ indicates that this system uses the kernel of FreeBSD,
  which by definition is always true on FreeBSD. This macro is also defined
  on other systems that use the kernel of FreeBSD, such as GNU/kFreeBSD.

  It is tempting to use this macro in userland code when we want to enable
  kernel-specific routines, and in fact it's fine to do this in code that
  is part of FreeBSD itself.  However, be aware that as presence of this
  macro is still not widespread (e.g. older FreeBSD versions, 3rd party
  compilers, etc), it is STRONGLY DISCOURAGED to check for this macro in
  external applications without also checking for __FreeBSD__ as an
  alternative.

Approved by:	kib (mentor)

git-svn-id: svn://svn.freebsd.org/base/stable/8@230432 ccf9f872-aa2e-dd11-9fc8-001c23d0bc1f