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.
Consistently include opt_ipsec.h for consumers of <netipsec/ipsec.h>.
This fixes ipsec.ko to include all of IPSEC_DEBUG.
99% of the time assert(3) will just compile out in userspace. Does this library have any other use of assert(3) (i.e. does it expect anybody to actually compile with -DDEBUG or whatever)?