fonbase: In firewall.fon, use a subshell for the 40-interzone script.
Previously, this script would be sourced. However, the script does some
checking and uses "exit 0" if it doesn't need to do any work, causing
the calling script (which is the /lib/firewall/uci_firewall.sh script)
to stop executing before the full firewall is loaded.
This issue could only occur when for some reason the wan network was
marked as being up when the Fonera was in bridge mode, so this shouldn't
really occur unless a user has been messing with network settings
manually. However, we'll fix this just to be sure.
References: #983
