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.

Reported by: jenkins (a lot), O. Hartmann (once)
Generally discussed with: imp, jhb