[multipathd] assorted cleanups

[multipathd] assorted cleanups

- remove the superfluous dm_mapname() "target type" param
- log more errors in the devmapper wrapper library
- in "struct event_thread" move from "pthread * thread" to "pthread thread"
- introduce a pthread_cleanup function
- wrap the waiter thread dmt in the "struct event_thread" for cleanup
- put cancelation points around the WAIT ioctl
- introduce a garbage collector to remove dead entries in the mpvec
- use strdup instead of asprintf as the later produce warnings here