Prevent invalid interface specifiers on queue rules

Prevent invalid interface specifiers on queue rules

pf.conf(5) states that queues attach to actual interfaces only, yet the
following parses:

	# echo queue eq on egress bandwidth 1G default | pfctl -f-
	# pfctl -sq
	pfctl: DIOCGETQSTATS: Bad file descriptor

	# echo queue rq on rdomain 0 bandwidth 1G default | pfctl -vf-
	queue rq bandwidth 1G default
	# pfctl -sq
	pfctl: DIOCGETQSTATS: Bad file descriptor

On rdomains, ifa_exists() returns NULL.
On interface groups, ifa_exists() returns non-NULL but af is never set
to AF_LINK.

OK henning sashan