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.

Change-Id: I0e7427274b018043c3a6a8ca6181a78a385d9aa2
Reviewed-on: http://gerrit.openafs.org/2864
Tested-by: BuildBot <buildbot@rampaginggeek.com>
Reviewed-by: Derrick Brashear <shadow@dementia.org>