Enable LaTeX-math-menu-unicode on w32 systems.

* latex.el (LaTeX-math-menu-unicode): Enable also on windows
systems as it seems to work there, too.
Fix BibTeX-auto-regexp-list.

* latex.el (BibTeX-auto-regexp-list): Remove `TeX-token-char' from
the regexp since cite keys can start with non-letter characters,
e.g., bibcodes start with year of publication.
Fix my mistake in previous commit.