Bring per-port LUN enable/disable code up to date:

• remove last remnants of never implemented multiple targets support;
• implement missing support for LUN mapping in this area.

Due to existing locking constraints LUN mapping code is practically
unlocked at this point. Hopefully it is not racy enough to live until
somebody get idea how to call sleeping fronend methods under lock also
taken by the same frontend in non-sleepable context. :(