FBSD: update to using bsd.kmod.mk

FBSD: update to using bsd.kmod.mk

We have for a long time had a cobbled-together kernel module build
system that essentially copied build arguments from a kernel build
in an ad hoc fashion, with lots of conditionals on architecture
and OS version.  (We got it wrong, several times, too.)
Instead, use the supported mechanism for building kernel modules,
which allows us to remove a lot of code from the Makefile and gives
us some measure of future-proofing.

FIXES 127578

Change-Id: Ibccea4507f57660becb4fb39a57b50c15ea4cb07
Reviewed-on: http://gerrit.openafs.org/5042
Reviewed-by: Garrett Wollman <wollman@csail.mit.edu>
Tested-by: Derrick Brashear <shadow@dementia.org>
Reviewed-by: Derrick Brashear <shadow@dementia.org>