pf: Apply kif flags to new group members

If we have a 'set skip on <ifgroup>' rule this flag it set on the group
kif, but must also be set on all members. pfctl does this when the rules
are set, but if groups are added afterwards we must also apply the flags
to the new member. If not, new group members will not be skipped until
the rules are reloaded.

Reported by: dvl@
Reviewed by: glebius@
Differential Revision: https://reviews.freebsd.org/D23254