If compiled in for dual-stack use, test with feature_present(3)
to see if we should register the IPv4/IPv6 address family related
options.
In case there is no "inet" support we would love to go with the
usage() and make the address family mandatory (as it is for anything
but inet in theory). Unfortunately people are used to
ifconfig IF up/down
etc. as well, so use a fallback of "link". Adjust the man page
to reflect these minor details.
Improve error handling printing a warning in addition to the usage
telling that we do not know the given address family in two places.
Reviewed by: hrs, rwatson
Sponsored by: The FreeBSD Foundation
Sponsored by: iXsystems
MFC after: 2 weeks
5af3fa9a5f