More FBSD syscall tweaking

More FBSD syscall tweaking

We're now properly registered in syscalls.master for HEAD
(i.e. proto-9.0) and RELENG_8 (proto-8.2), which means that
afs3_syscall is prototyped in sys/sysproto.h .  Accordingly,
don't declare it in afs_prototypes.h for those cases.

Also add FBSD82_ENV checks for the new syscall-registration code,
and cast afs3_syscall to sy_call_t* for the sysent structure.

Reviewed-on: http://gerrit.openafs.org/2864
Tested-by: BuildBot <buildbot@rampaginggeek.com>
Reviewed-by: Derrick Brashear <shadow@dementia.org>
(cherry picked from commit 791b7a4c660ef468286eb6f51e66b4faa4bb6c7b)
Change-Id: I6c58f2f08dafa5a0db98febde8f2fcb6781225e1
Reviewed-on: http://gerrit.openafs.org/2932
Tested-by: Derrick Brashear <shadow@dementia.org>