Rename enum to OPERATION_LABEL_FILESYSTEM (#741424)

This and the following few commits rename variables, methods, classes,
etc from *label_partition* to *label_filesystem* so that the code also
reflects that it is the label of the file system that is being modified
and to separate it from the name partition operation about to be added.

enum OPERATION_LABEL_PARTITION -> OPERATION_LABEL_FILESYSTEM

Bug 741424 - Add support for GPT partition names