fqdn: Skip "open ports" check for statically configured ports

[gandro]

When restoring the previous DNS proxy port, we check if the port is already in-use. However, if the port we retreived from GetProxyPort was previously set via SetProxyPort, then we want to use it unconditionally. We rely on isStatic for this, as as static ports cannot change and the open port may be open with SO_REUSEPORT (which proxy.OpenLocalPorts() does not check). Restored ports never have isStatic set to true, so this does retain the "open ports" check if the port was restored.

In addition, when restoring ports we want to make sure that previous calls to SetProxyPort are also not overwritten, thus this commit also only restores the port if it wasn't explicitly set.

This is the same behavior we had previously, which did not check the returned port of d.l7Proxy.GetProxyPort against the list of open ports.

Fixes: d11e4d261279 ("proxy: Reuse proxy ports from datapath on restart")

[gandro]

/test

[gandro]

/test

[gandro]

CI is green except for https://github.com/cilium/cilium/issues/33257 which is expected. Merging.