pf: Fix 'set skip on' for groups

The pfi_skip_if() function sometimes caused skipping of groups to work,
if the members of the group used the groupname as a name prefix.
This is often the case, e.g. group lo usually contains lo0, lo1, ...,
but not always.

Rather than relying on the name explicitly check for group memberships.

Obtained from: OpenBSD (pf_if.c,v 1.62, pf_if.c,v 1.63)
Sponsored by: Essen Hackathon