FBSD: deal with kernel API rename

FBSD: deal with kernel API rename

Upstream decided to rename the kernel functions that implement
syscalls to have a sys_prefix (including afs3_syscall!).
We use a couple of them, so we need to conditionalize accordingly.
Unfortunately, __FreeBSD_version was not bumped with the change,
so we use something close to it and hope it's close enough.

Change-Id: I228805a813af70b30f94f659e4da602210a6d6df
Reviewed-on: http://gerrit.openafs.org/5572
Reviewed-by: Derrick Brashear <shadow@dementix.org>
Tested-by: Derrick Brashear <shadow@dementix.org>