fix: open GUI when relaunched instead of printing error (fixes #10727) (#10852)

This is primarily to improve the experience in environments that start
Syncthing from a graphical environment, e.g., Windows.

When already running, instead of printing an error we open the GUI. If
the GUI is not available (but the lockfile indicates we are running),
print an error and exit.

If --no-browser is given or STNOBROWSER is set, act like before.

Closes #10736. Marking as `fix` because this does not deserve to trigger
a minor release on its own.

---------

Signed-off-by: Jakob Borg <jakob@kastelo.net>

lib/svcutil/svcutil.go

@@ -88,6 +88,7 @@ const (
 	ExitNoUpgradeAvailable ExitStatus = 2
 	ExitRestart            ExitStatus = 3
 	ExitUpgrade            ExitStatus = 4
+	ExitNoRestart          ExitStatus = 5 // do not get restarted by monitor
 )
 
 func (s ExitStatus) AsInt() int {