sound: Make device cloning work

sound: Make device cloning work

* Some lockstatus() calls were wrong and needed to be compared
  with LK_EXCLUSIVE

* The sound device caches dynamically created devices and tries
  to reuse them, but devfs got angry if the clone code passed
  it a device which was already in the filesystem namespace. devfs
  had to be modified in 2c94b9eeb85c6cfa5ff918aa58ce484b27fc7c91
  and eb1c7cbc9e1052820efc6b0b7a94c40a7d45b329

* The clone code was designed to clone to new unit numbers, not
  clone an existing unit into multiple channels