nfsmount: fix --volname being ignored rclone#7503

Before this change, nfsmount ignored the --volname flag. After this change, the --
volname flag is respected, making it possible to set a custom volume name.

macOS users should note that Finder will show the correct volume name in most
places, but a notable exception is the sidebar, which will show "localhost".
This seems to be a system limitation (at least without `sudo`), but see the
discussion at rclone#7503 (comment)
for some possible workarounds.

cmd/nfsmount/nfsmount.go

@@ -70,7 +70,7 @@ func mount(VFS *vfs.VFS, mountpoint string, opt *mountlib.Options) (asyncerrors
 	}
 	cmd = append(cmd, "mount")
 	cmd = append(cmd, options...)
-	cmd = append(cmd, "localhost:", mountpoint)
+	cmd = append(cmd, "localhost:"+opt.VolumeName, mountpoint)
 	fs.Debugf(nil, "Running mount command: %q", cmd)
 
 	out, err := exec.Command(cmd[0], cmd[1:]...).CombinedOutput()