pf: allow unspecified addressed for certain MLD messages

As per RFC 3590 MLD Report and Done messages are permitted to use the
unspecified address as a source address (e.g. during duplicate address
detection for the first IPv6 address). Allow this, but only this.

Reported by: Alexander Leidinger <Alexander@Leidinger.net>
Reviewed by: bms
See also: OpenBSD, sashan <sashan@openbsd.org>, 60036e8507
Sponsored by: Rubicon Communications, LLC ("Netgate")
Differential Revision: https://reviews.freebsd.org/D59334