networkmanager: Wait for iptables lock in shared dispatcher script

[mtoman]

The dispatcher script that moves around FORWARD rules of shared interfaces currently calls iptables assuming it will always work but in practice two iptables commands can not run in parallel and we have occasionally seen the script fail with:

Another app is currently holding the xtables lock. Perhaps you want to use the -w option?

This patch adds the -w option to make the script wait for the lock when necessary.

It also makes the script exit with an error code if anything fails, which will log the output as WARN instead of INFO as the errors are easy to overlook at this moment.

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