Remove dependence on source tree options. Move all kernel module
options into kern.opts.mk and change all the places where we use
src.opts.mk to pull in the options. Conditionally define SYSDIR and
use SYSDIR/conf/kern.opts.mk instead of a CURDIR path. Replace all
instances of CURDIR/../../etc with STSDIR, but only in the affected
files.

As a special compatibility hack, include bsd.owm.mk at the top of
kern.opts.mk to allow the bare build of sys/modules to work on older
systems. If the defaults ever change between 9.x, 10.x and current for
these options, however, you'll wind up with the host OS' defaults
rather than the -current defaults. This hack will be removed when
we no longer need to support this build scenario.

Reviewed by: jhb
Differential Revision: https://phabric.freebsd.org/D529