pfctl: Fix ifgroup check

We cannot just assume that any name which ends with a letter is a group
That's not been true since we allowed renaming of network interfaces. It's also
not true for things like epair0a.

Try to retrieve the group members for the name to check, since we'll get ENOENT
if the group doesn't exist.

MFC after: 1 week
Event: Aberdeen hackathon 2019