fix(config): respect GUI address override in fresh default config (fixes #9783) (#9675)

### Purpose

When generating a new `config.xml` file with default options, the GUI
address is populated with a hard-coded default value of
`127.0.0.1:8384`, except for a random free port if that default one is
occupied. This is independent from the GUI configuration default address
defined in the protobuf description. More importantly, it ignores any
`STGUIADDRESS` override given via environment variable or command-line
option, thus probing for the default port instead of the one specified
via override.

The `ProbeFreePorts()` function now respects the override, by reading
the `GUIConfiguration.Address()` method instead of using hard-coded
defaults.

When not calling `ProbeFreePorts()`, the override should still be
persisted rather than the default address. This happens only when
generating a fresh default `config.xml`, never on an existing one.

lib/syncthing/utils.go

@@ -63,6 +63,8 @@ func DefaultConfig(path string, myID protocol.DeviceID, evLogger events.Logger,
 
 	if skipPortProbing {
 		l.Infoln("Using default network port numbers instead of probing for free ports")
+		// Record address override initially
+		newCfg.GUI.RawAddress = newCfg.GUI.Address()
 	} else if err := newCfg.ProbeFreePorts(); err != nil {
 		return nil, err
 	}