network-functions.pl: fix network membership test

network-functions.pl: fix network membership test

This is based on an orphaned patch provided by Tim FitzGeorge and
_finally_ fixes incorrect network membership calculations. Those were
are usability pain in the ass deluxe, as they rendered some combinations
of configuring OpenVPN and IPsec services unusable.

Fixes: #11235
Fixes: #12263
Cc: Tim FitzGeorge <ipfr@tfitzgeorge.me.uk>
Cc: Michael Tremer <michael.tremer@ipfire.org>
Cc: Alexander Marx <alexander.marx@ipfire.org>
Signed-off-by: Peter Müller <peter.mueller@ipfire.org>
Signed-off-by: Arne Fitzenreiter <arne_f@ipfire.org>