Use feature_present(3) to determine whether to open an INET or an
INET6 socket when needed to allow pfctl to work on noinet and noinet6
kernels (and try to provide a fallback using AF_LINK as best effort).
Adjust the Makefile to also respect relevant src.conf(5) options
for compile time decisions on INET and INET6 support.
Reviewed by: glebius (no objections)
f57b2f8c40