Add dm_task_get_name_mangled/unmangled to libdevmapper.

Add dm_task_get_name_mangled/unmangled to libdevmapper.

dm_task_get_name_mangled will always return mangled form of the name while
the dm_task_get_name_unmangled will always return unmangled form of the name
irrespective of the global setting (dm_set/get_name_mangling_mode).

This is handy in situations where we need to detect whether the name is already
mangled or not. Also display functions make use of it.