pfctl: Zap bits in host_v4(), use mask parameter

This avoids a duplicate strrchr() call and makes the function consistent
with host_v6() regarding mask handling.

While here, use the destination's size in memcpy instead of hardcoding its
type.

OK sashan

Obtained from: OpenBSD, kn <kn@openbsd.org>, a7ede25358
Sponsored by: Rubicon Communications, LLC ("Netgate")