Currently we log on every single one of 10 retries deep in the upnp
stack. However we also return the failure as an error, which is bubbled
up a while until it's logged at debug level. Switch that around, such
that the repeat logging happens at debug level but the top-level happens
at info. There's some chance that this will newly log errors from
nat-pmp that were previously hidden in debug level - I hope those are
useful and not too numerous.
Also potentially this can even close#9324, my (very limited)
understanding of the reports/discussion there is that there's likely no
problem with syncthing beyond the excessive logging, it's some weird
router behaviour.
This pull request allows syncthing to request an IPv6
[pinhole](https://en.wikipedia.org/wiki/Firewall_pinhole), addressing
issue #7406. This helps users who prefer to use IPv6 for hosting their
services or are forced to do so because of
[CGNAT](https://en.wikipedia.org/wiki/Carrier-grade_NAT). Otherwise,
such users would have to configure their firewall manually to allow
syncthing traffic to pass through while IPv4 users can use UPnP to take
care of network configuration already.
### Testing
I have tested this in a virtual machine setup with miniupnpd running on
the virtualized router. It successfully added an IPv6 pinhole when used
with IPv6 only, an IPv4 port mapping when used with IPv4 only and both
when dual-stack (IPv4 and IPv6) is used.
Automated tests could be added for SOAP responses from the router but
automatically testing this with a real network is likely infeasible.
### Documentation
https://docs.syncthing.net/users/firewall.html could be updated to
mention the fact that UPnP now works with IPv6, although this change is
more "behind the scenes".
---------
Co-authored-by: Simon Frei <freisim93@gmail.com>
Co-authored-by: bt90 <btom1990@googlemail.com>
Co-authored-by: André Colomb <github.com@andre.colomb.de>
