Thin pool improvements

Rebased ontop of @rhvgoyal's changes.  This pull request provides:
1) small udev scanning fix for the case where docker creates the thin-pool
2) new --storage-opt dm.thinpooldev to allow an admin to create/manage the thin-pool that docker uses outside of docker.
can we have an end to end, detailed and explained example of the 'best practice to date' for this?

(also - these option descriptions are now in the deamon man page, and the cli docs (mmm, or will be soon?)

the example probably deserves to be under the `run.md` doc for now