MFC 361638,361712: Only build ipsec modules for kernels with IPSEC_SUPPORT.

361638:
Only build ipsec modules if the kernel includes IPSEC_SUPPORT.

Honoring the kernel-supplied opt_ipsec.h in r361632 causes builds of
ipsec modules to fail if the kernel doesn't include IPSEC_SUPPORT.
However, the module can never be loaded into such a kernel, so only
build the modules if the kernel includes IPSEC_SUPPORT.

361712: (kevans)
modules: don't build ipsec/tcpmd5 if the kernel is configured for IPSEC

IPSEC_SUPPORT can currently only cope with either IPSEC || IPSEC_SUPPORT,
not both. Refrain from building if IPSEC is set, as the resulting module
won't be able to load anyways if it's built into the kernel.

KERN_OPTS is safe here; for tied modules, it will reflect the kernel
configuration. For untied modules, it will defer to whatever is set in
^/sys/conf/config.mk, which doesn't set IPSEC for modules. The latter
situation has some risk to it for uncommon scenarios, but such is the life
of untied kernel modules.

MFC 361632: Honor opt_ipsec.h from kernel builds.

To make this simpler, set the default contents of opt_ipsec.h
for standalone modules in sys/conf/config.mk.

MFC r303195:

Don't run find(1) for __MPATH with NO_MODULES set.

Approved by: re (gjb)