pf: should be enforcing TTL=1 to packets sent to 224.0.0.1 only.

Issue found and kindly reported by Luca Di Gregorio <lucdig _at_ gmail>

OK bluhm@

Obtained from: OpenBSD, sashan <sashan@openbsd.org>, 58feb3ffc6
Sponsored by: Rubicon Communications, LLC ("Netgate")