pf: attempt to handle overlapping group and interface names

pf assumes that network groups and network interfaces share a namespace
(that is, a name is unused, a group or an interface, never both a the
same time). Unfortunately this assumption was broken when interface
renaming was introduced.
Attempt to cope with this rather than panicking. Note that this is a
band-aid, not a full solution. The correct fix is for the network stack
to go back to enforcing a single namespace for groups and interfaces.

PR: 297220
Reported by: Robert Morris
MFC after: 1 week
Sponsored by: Rubicon Communications, LLC ("Netgate")