pfctl: improve bitmask printing

In some cases the netmask gets set to a full 128 bit mask even if no
address family is selected; don't print the v6 mask if it's a v4 address.

Obtained from: OpenBSD, mcbride <mcbride@openbsd.org>, f6d7b41f07
Sponsored by: Rubicon Communications, LLC ("Netgate")
Differential Revision: https://reviews.freebsd.org/D46584