Provide a way for multiple libraries to synchronize use of libdevmapper

[DemiMarie]

Since libdevmapper isn’t thread-safe, multi-threaded users of it need to be able to synchronize their use of libdevmapper. A simple and robust way to do this is to have libdevmapper provide global functions dm_lock and dm_unlock, which respectively acquire and release a mutex. Libraries would need to call dm_lock before performing any thread-unsafe operations, and dm_unlock afterwards.