Support --mount-master option of SuperSU's su

[rutsky]

Recent versions of SuperSU use mount namespace separation that makes new mount points visible only inside su shell session.
This behavior can be disabled using --mount-master option or globally in SuperSU settings.
Disabling of mount namespace separation is desired behavior for instance for applications that mounts external devices (e.g. https://github.com/acomminos/Mountie).