Update to the new thread world order for FBSD

Update to the new thread world order for FBSD

Since FreeBSD 5.0, system calls have taken a thread argument instead
of a proc argument.  Finish catching up.

Also remove the retval argument, which is not in the system sysent
definition, and is unused.

Since we took our sweet time getting around to it, we can also
remove large swaths of FreeBSD 4.X-only code from the area.

Fix the vop_*_args struct definition comments that had changes in
addition to proc-->thread while we're here.

Change-Id: Ib469e77655a6c04bfbfe1e5fab54f6f3a6119a85
Reviewed-on: http://gerrit.openafs.org/1675
Reviewed-by: Derrick Brashear <shadow@dementia.org>
Tested-by: Derrick Brashear <shadow@dementia.org>