Make devicemapper initialization happen on startup

We used to initialize devicemapper lazily on first use, because
at that point there was also support for aufs and at runtime init
time we didn't know which backend was to be used. So, we just
always created a deviceset and relied on it doing nothing if it
wasn't used.

These days we only use devmapper, and doing the init lazily is
problematic because if devmapper fails nothing will work, and
we want to see that immediately, rather than each devmapper
call failing when we start using docker.