devmapper: Move all "raw" libdevmapper wrappers to devmapper.go

This separates out the DeviceSet logic a bit better from the raw
device mapper operations.

devicemapper: Serialize addess to the devicemapper deviceset

This code is not safe to run in multiple threads at the same time,
and neither is libdevmapper.

DeviceMapper: Move deactivate into UnmountDevice

This way the deactivate is atomic wrt othe device mapper operations
and will not fail with EBUSY if someone else starts a devicemapper
operation inbetween unmount and deactivate.

devmapper: Fix loopback mounting regression

Some changes were added to attach_loop_device which added
a perror() in a place that caused it to override errno so that
a later errno != EBUSY failed. This fixes that and cleans up
the error reporting a bit.

devmapper: Build on old kernels without LOOP_CTL_GET_FREE define