New API: Bind the tune2fs command.

Previously we bound the 'tune2fs -l' command so that we could list out
the tunables of an ext2/3/4 filesystem.  Also commands like
set_e2label and set_e2uuid used tune2fs.

This commit binds many of the tunables that can be set using tune2fs.

The coverage is not complete, but we can add more later because this
uses optional parameters so the call is extensible without breaking
ABI.  The current change gives us enough for using libguestfs within
OpenStack.