vfs: fix SIGHUP killing serve instead of flushing directory caches

Before, rclone serve would crash when sent a SIGHUP which contradicts
the documentation - saying it should flush the directory caches.

Moved signal handling from the mount into the vfs layer, which now
handles SIGHUP on all uses of the VFS including mount and serve.

Fixes #8607

cmd/mountlib/mount.go

@@ -403,27 +403,7 @@ func (m *MountPoint) Wait() error {
 	fnHandle := atexit.Register(finalise)
 	defer atexit.Unregister(fnHandle)
 
-	// Reload VFS cache on SIGHUP
-	sigHup := make(chan os.Signal, 1)
-	NotifyOnSigHup(sigHup)
-	var err error
-
-	waiting := true
-	for waiting {
-		select {
-		// umount triggered outside the app
-		case err = <-m.ErrChan:
-			waiting = false
-		// user sent SIGHUP to clear the cache
-		case <-sigHup:
-			root, err := m.VFS.Root()
-			if err != nil {
-				fs.Errorf(m.VFS.Fs(), "Error reading root: %v", err)
-			} else {
-				root.ForgetAll()
-			}
-		}
-	}
+	err := <-m.ErrChan
 
 	finalise()