pfctl: match broadcast address behaviour to the kernel

When using a /32 IPv4 interface address, pfctl(8) reports a 0.0.0.0
as broadcast address. The kernel does not consider this a broadcast
address and ifconfig(8) has a check to exclude it. Use the same
check in pfctl(8).
Found by regress/sbin/pfctl pfi2; OK mikeb@

Obtained from: OpenBSD, bluhm <bluhm@openbsd.org>, cf181e999d
Sponsored by: Rubicon Communications, LLC ("Netgate")