gpt: move gpt guid printing function to common library
gpt: switch partition names to a 16 bit type

In UEFI/GPT strings are UTF-16 so use a uint16 to make dealing with the
string practical.
tests: add some partitions to the gpt unit test data
gpt: add search by partition label and uuid commands

Builds on the existing filesystem search code. Only for GPT right now.
Merge pull request #7 from marineam/gptprio

Generic GPT partition search commands