daemon: parted: refactor sgdisk info parsing code

Isolate in an own function the code that runs sgdisk and parse a field
of it (using an extraction function passed as parameter), using it for
the GUID type.

This is just code motion, no actual behaviour changes.
New API: part-get-name (RHBZ#593511).

Counterpart of part-set-name, it uses sgdisk (hence needs the "gdisk"
feature) to query for the label/name of a partition in a GPT layout.
resize: preserve GPT partition names (RHBZ#1060404).

Save the partition names/labels of the source partitions, and restore
them after the partition copy.