BUG11559: firewall-lib

BUG11559: firewall-lib

When creating firewallrules or using firewall groups,
it should be possible to select a single IpSec subnet if there is more than one.

This patch has neccessary changes for the firewall-lib. While the network name of the IpSec changes
on save (subnet is added to name) we need to split the name or normalise the field before using it.
Signed-off-by: Alexander Marx <alexander.marx@ipfire.org>
Tested-by: Peter Müller <peter.mueller@link38.eu>
Signed-off-by: Michael Tremer <michael.tremer@ipfire.org>