tunspace: bunch of improvements

[pktpls]

Compile tested: ipq40xx, ath79
Run tested: Fritzbox 4040, Glinet GL-XE300

Description of your changes:

These changes came about while setting up the XE300's internal LTE modem.

• Handling of the uplink interface in various difficult situations is now much more robust, it recovers more reliably, etc.
• Some types of uplink interfaces can't be added to a bridge, for various reasons. We now support a "direct" uplink mode which doesn't clone a macvlan bridge interface, and instead moves the uplink interface itself directly to the namespace.
  • The downside of this mode is that the fully configured and functional uplink interface will definitely leak out of the namespace on tunspace restart or stop.
• The wg-installer servers on each gateway have a timeout based on a client's private key, which rejects reconnections (as in: re-registration) for 10 minutes or so. This can easily lead to situations where all available servers reject our private key for an annoyingly long time.
  • The tunspace client now generates a fresh new private key whenever it registers with a server. This circumvents the re-registration timeout, and there's also really no need for the wireguard keys to be permanent.

[Akira25]

Looks good to me.

For the Server-Timeout: Is there a good reason, why not abolishing that timeout?

[Akira25]

The CI got fixed in #441. You may rebase on the current master branch.

[pktpls]

For the Server-Timeout: Is there a good reason, why not abolishing that timeout?

I just dont want to touch wg-installer unless I must :) And the timeout does have its purpose (cleaning up unused interfaces), it's just that it should not reject reconnections.

[pktpls]

also backported to openwrt-23.05 branch (along with 355a923)