[Libguestfs] [PATCH v2] Add tune2fs command.

The changes since the previous patch: - safe ADD_ARG macro for adding arguments to a fixed size stack array - support for testing functions that return RHashtable, ie. tune2fs-l. - add tests that set (tune2fs) and get (tune2fs-l) various parameters. - only one 'intervalbetweenchecks' parameter (in seconds) Rich.