all: Use context in lib/dialer (#6177)

* all: Use context in lib/dialer

* a bit slimmer

* https://github.com/syncthing/syncthing/pull/5753

* bot

* missed adding debug.go

* errors.Cause

* simultaneous dialing

* anti-leak

lib/util/utils.go

@@ -236,6 +236,9 @@ func (s *service) Serve() {
 
 	var err error
 	defer func() {
+		if err == context.Canceled {
+			err = nil
+		}
 		s.mut.Lock()
 		s.err = err
 		close(s.stopped)