mount: add --devname to set the device name sent to FUSE for mount display

Before this change, the device name was always the remote:path rclone
was configured with. However this can contain sensitive information
and it appears in the `mount` output, so `--devname` allows the user
to configure it.

See: https://forum.rclone.org/t/rclone-mount-blomp-problem/29151/11

cmd/mountlib/utils.go

@@ -87,7 +87,7 @@ func (m *MountPoint) CheckAllowings() error {
 // SetVolumeName with sensible default
 func (m *MountPoint) SetVolumeName(vol string) {
 	if vol == "" {
-		vol = m.Fs.Name() + ":" + m.Fs.Root()
+		vol = fs.ConfigString(m.Fs)
 	}
 	m.MountOpt.SetVolumeName(vol)
 }
@@ -102,3 +102,11 @@ func (o *Options) SetVolumeName(vol string) {
 	}
 	o.VolumeName = vol
 }
+
+// SetDeviceName with sensible default
+func (m *MountPoint) SetDeviceName(dev string) {
+	if dev == "" {
+		dev = fs.ConfigString(m.Fs)
+	}
+	m.MountOpt.DeviceName = dev
+}