pygrub: further improve grub2 support

pygrub: further improve grub2 support

      * Improve syntax error messages to say what actually went wrong
        instead of giving an arbitrary and basically useless
        integer.
      * Improve handling of quoted values used with the "set" command,
        previously only the default variable was special cased to
        handle quoting.
      * Allow for extra options to the menuentry command, syntax now
        appears to be
                        menuentry "TITLE" --option1 --option2 {...}

Signed-off-by: Ian Campbell <ian.campbell@citrix.com>