MFC 226217,227070,227341,227502:

MFC 226217,227070,227341,227502:
Add the posix_fadvise(2) system call.  It is somewhat similar to
madvise(2) except that it operates on a file descriptor instead of a
memory region.  It is currently only supported on regular files.

Note that this adds a new VOP, so all filesystem modules must be
recompiled.

Approved by:	re (kib)

git-svn-id: svn://svn.freebsd.org/base/stable/8@229725 ccf9f872-aa2e-dd11-9fc8-001c23d0bc1f