DEVFS - Rollup

DEVFS - Rollup

* Use a dynamically sized dirent structure for devfs_nodes, saving a lot
  of memory.

* Fix m_* lexical space overloading issues.

* Fix issues with numerous uninitialized fields.

* Completely initialize the devfs_node before making it visible to
  userland.

* Properly invalidate negative entries in the namecache when adding new
  devfs nodes.  Also properly recycle disconnected vnodes.

* Properly disassociate a devfs node from its vnode when freeing the
  devfs node.

* Clarify device reference counts and fix a bug or two.

* Get reprobes working.  Sniff writes and automatically reprobe the
  whole disk or a particular slice on close() if the MBR or label area
  is modified.

* Fix memory leak when mounting/umounting devfs.

Submitted-by: Alex Hornung <ahornung@gmail.com>