initscripts: Always wait for xtables lock when running iptables commands

initscripts: Always wait for xtables lock when running iptables commands

If not explicitly instructed to do so, iptables by default aborts with
an error message such as

> Can't lock /run/xtables.lock: Resource temporarily unavailable
> Another app is currently holding the xtables lock. Perhaps you want to use the -w option?

if the Xtables lock is still set, i.e., another iptables operation is
currently in progress. This causes iptables commands not to be executed
at all if there are delays during the boot procedure, e.g. due to slow
PPPoE dial-up procedure or similar.

To ensure deterministic behavior, this match modifies initscripts to
always execute iptables to wait for the Xtables lock to be removed, to
make sure iptables rules are installed properly (the "firewall"
initscript is doing so already).

Fixes: #13896 - OpenVPN RW port not opened in firewall after reboot
Signed-off-by: Peter Müller <peter.mueller@ipfire.org>
Tested-by: Peter Müller <peter.mueller@ipfire.org>
Signed-off-by: Michael Tremer <michael.tremer@ipfire.org>