Improve ACPI hotplug handling such that dock event is handled properly.

* Register handlers for dock events.

* Directly detach device on EJECT_REQUEST instead of signaling hotplug
  event.  This prevents libata from accessing severed controller
  and/or device.

* While at it, use named constants for ACPI events and move uevent
  signaling inside host lock.

Original patch and testing by Holger Macht.
Signed-off-by: Tejun Heo <htejun@gmail.com>
Cc: Holger Macht <hmacht@suse.de>
Signed-off-by: Jeff Garzik <jeff@garzik.org>