Don't pass nil to mmm-set-local-variables

Finishes e848ef1.
Simplify mmm-update-mode-info

* Use non-hidden name for the temporary buffer, for better font-lock
  initialization (and for minor modes that depend on it, like whitespace-mode).
* Kill the temp buffer in an unwind form, error or not.
* Don't check if it's XEmacs version 20, 21 came out 14 years ago.
* Get rid of `mmm-font-lock-available-p' and `mmm-set-font-lock-defaults'. The
  former was only used with Emacs < 21, the latter is just unused now.
Allow toggling whitespace-mode after initialization

Refs #5
Merge pull request #19 from dgutov/font-lock

Allow toggling whitespace-mode