rules.pl: Allow dynamic destory of loaded but unused ipset sets.

rules.pl: Allow dynamic destory of loaded but unused ipset sets.

Instead of stupidly destroying all ipsets, we now grab the already loaded sets
and compare them with the loaded sets during runtime of the script.

So we are now able to determine which sets are not longer required and
safely can destroy (unload) at a later time.

This saves us from taking care about dropping/flushing rules which are
based on ipset before we can destroy them - because only unused sets are
affected.

Signed-off-by: Stefan Schantl <stefan.schantl@ipfire.org>
Inspired-by: Tim FitzGeorge <ipfr@tfitzgeorge.me.uk>
Reviewed-by: Michael Tremer <michael.tremer@ipfire.org>