Avoid bailing out early if multiple instances of iptables-restore are
called simultaneously.
FixesQubesOS/qubes-issues#3665
(cherry picked from commit 8f6bd245bd)
There were multiple problems with reusing existing one:
- need to sync with upstream changes (configuration path etc)
- conflicts resolution on updates
- lack of iptables --wait, which causes firewall fail to load sometimes
QubesOS/qubes-issues#1067