[lib] abstract a little more devmapper internals

[lib] abstract a little more devmapper internals

o move DEFAULT_TARGET define from defaults.h to devmapper.h
o rename DEFAULT_TARGET into TGT_MPATH (multipath)
o introduce TGT_PART (linear)
o remove the type param from functions used only with TGT_MPATH
o abstract dm_addmap() with to wrappers dm_addmap_create() and
  dm_addmap_reload(). Wrappers don't require the type and task
  params.
o move the dm_addmap(DM_DEVICE_CREATE, ...) cleanup on failure
  from configure.c into devmapper.c::dm_addmap_create()