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/mount.go

@@ -40,6 +40,7 @@ type Options struct {
 	ExtraOptions       []string
 	ExtraFlags         []string
 	AttrTimeout        time.Duration // how long the kernel caches attribute for
+	DeviceName         string
 	VolumeName         string
 	NoAppleDouble      bool
 	NoAppleXattr       bool
@@ -125,6 +126,7 @@ func AddFlags(flagSet *pflag.FlagSet) {
 	flags.BoolVarP(flagSet, &Opt.AsyncRead, "async-read", "", Opt.AsyncRead, "Use asynchronous reads (not supported on Windows)")
 	flags.FVarP(flagSet, &Opt.MaxReadAhead, "max-read-ahead", "", "The number of bytes that can be prefetched for sequential reads (not supported on Windows)")
 	flags.BoolVarP(flagSet, &Opt.WritebackCache, "write-back-cache", "", Opt.WritebackCache, "Makes kernel buffer writes before sending them to rclone (without this, writethrough caching is used) (not supported on Windows)")
+	flags.StringVarP(flagSet, &Opt.DeviceName, "devname", "", Opt.DeviceName, "Set the device name - default is remote:path")
 	// Windows and OSX
 	flags.StringVarP(flagSet, &Opt.VolumeName, "volname", "", Opt.VolumeName, "Set the volume name (supported on Windows and OSX only)")
 	// OSX only
@@ -235,6 +237,7 @@ func (m *MountPoint) Mount() (daemon *os.Process, err error) {
 		return nil, err
 	}
 	m.SetVolumeName(m.MountOpt.VolumeName)
+	m.SetDeviceName(m.MountOpt.DeviceName)
 
 	// Start background task if --daemon is specified
 	if m.MountOpt.Daemon {