Generic /cmd endpoint and dm specific operations (resize/resize-pool/trim-pool)

This series adds a generic /cmd endpoint and cli support for it, so that drivers and other form of plugins can add arbitrary new operations, which are accessible as "docker <plugin>:<operation> args..." .

Additionally it adds devicemapper specific operations `dm:resize-pool`, `dm:resize` and `dm:trim-pool`. These allow resizeing the devicemapper pool, indivdual devicemapper devices (like images and containers) and finally allows you to return unused space in the pool to the host root filesystem (when using loopback).

The last one is interesting because it lets you get space back if the discard trick we're using on delete doesn't work (older kernel) or is disabled (because its very slow).