dm - Modularize it fully

dm - Modularize it fully

* Move each of the targets (except error and zero) into a subdirectory
  in targets/.

* Change the macro DM_TARGET_MODULE to also let the module depend on dm.

* Add a new macro DM_TARGET_BUILTIN, which must be used instead of
  DM_TARGET_MODULE for targets that are compiled into the main dm.ko
  instead of a separate module. Not doing this will prevent dm.ko from
  unloading.

* Enable automatic loading of target modules if they aren't builtin or
  loaded yet. Automatically loaded modules can be unloaded via kldunload
  safely via a little hack (see source).

* NOTE: automatic loading doesn't always work. /boot must be mounted
  already, so if you are planning on encrypting your swap or similar
  stuff, you need to *EXPLICITLY* load dm_target_crypt.ko via the
  loader.

* NOTE: PLEASE NOTE THE NOTE ABOVE!

* Fix a little issue with dm_target_crypt that would print "unloaded"
  even if it wouldn't unload.

* Change the name of dm_target_stripe to dm_target_striped, since the
  target is actually called "striped".

* Move dm.h to sys/sys/dm.h

* Get rid of files.dm, a leftover from the import