Make the seek a method of the struct fileops.

Tested by:	pho
Sponsored by:	The FreeBSD Foundation
Implement read(2)/write(2) and neccessary lseek(2) for posix shmfd.
Add MAC framework entries for posix shm read and write.

Do not allow implicit extension of the underlying memory segment past
the limit set by ftruncate(2) by either of the syscalls.  Read and
write returns short i/o, lseek(2) fails with EINVAL when resulting
offset does not fit into the limit.

Discussed with:	alc
Tested by:	pho
Sponsored by:	The FreeBSD Foundation
Add simple test for the read/write/lseek on posix shm filedescriptor.

Tested by:	pho
Sponsored by:	The FreeBSD Foundation
Revert r254520 and resurrect the M_NOFREE mbuf flag and functionality.

Requested by:	np, grehan