Add go native device mapper library

[chrisccoulson]

I'm currently working on some code that is querying the device mapper subsystem using the dmsetup command. I'm not a huge fan of parsing commandline output and believe this sort of thing inevitably results in regressions later on as the format of commandline arguments generally don't have stability guarantees. They are also affected by locale.

The device mapper subsystem is driven using ioctls on /dev/mapper/control and is fairly well documented in the kernel. It shouldn't be too hard for us to provide a native go library that does this for us.