refactor(cgpt): Move blkid helper code to a common file.

These functions will be useful for some other code. Not adding to
cgpt_common.c since that gets compiled into a shared library I don't
want to link libblkid to.
feat(cgpt): Support passing partition device nodes to add and show.

Instead of always requiring the whole disk device and the partition
number cgpt can now look up that info itself if the device passed to it
is a single partition instead of the whole disk. This should make a
number of scripts using cgpt simpler.
Merge pull request #13 from marineam/blkid

support passing partition device nodes to cgpt