rules.pl: apply location filter to ppp0 if configured

rules.pl: apply location filter to ppp0 if configured

In order to prevent collateral damage to internal traffic, commit
c69c820025c21713cdb77eae3dd4fa61ca71b5fb introduced applying location
block on red0 as a sanity check.

On systems configured to use PPPoE, however, traffic appears on the ppp0
interface instead. This patch checks if a system is configured to use
this connection method, and applies the location filter to this
interface. red0 is used otherwise.

Fixes: #12519
Cc: Michael Tremer <michael.tremer@ipfire.org>
Signed-off-by: Peter Müller <peter.mueller@ipfire.org>
Signed-off-by: Michael Tremer <michael.tremer@ipfire.org>