FBSD: properly register our syscall

FBSD: properly register our syscall

Use the provided interface, syscall_register(), instead of
manually tweaking the sysent table.
Starting afsd will still fail at present on FreeBSD HEAD
without an additional kernel patch to syscalls.master.

Change-Id: I18db7c8b74dbd114165f7617df25e462cb372e88
Reviewed-on: http://gerrit.openafs.org/2616
Reviewed-by: Derrick Brashear <shadow@dementia.org>
Tested-by: Derrick Brashear <shadow@dementia.org>
(cherry picked from commit 5b71bab4ff0a0f6f1e0cec2c71adfd48b3f35cca)
Reviewed-on: http://gerrit.openafs.org/2679