pfctl: Merge host_v{4,6}() into simpler host_ip()

Except for networks such as "10/8" host_ip() now handles addresses in an
AF-agnostic way with more duplicate code removed/merged.

OK sashan (as for earlier copy_satopfaddr() diff) henning

Obtained from: OpenBSD, kn <kn@openbsd.org>, 1b6fdc2f24
Sponsored by: Rubicon Communications, LLC ("Netgate")