Fix some runtime issues in XEmacs.

* latex.el (TeX-latex-mode): Add second argument to
`local-variable-p', mandatory in XEmacs.  Suggested by Ikumi
Keita.

* preview/preview.el (preview-dump-state): Ditto.

* style/biblatex.el ("biblatex"): Ditto.

* tex.el (TeX-how-many): Make the function return a number also in
XEmacs and Emacs 21.  Suggested by Ikumi Keita.