Fix some XEmacs compilation issues.

* tex-buf.el (TeX-error-description-error): Make face definition
XEmacs compatible.
(TeX-error-description-tex-said): Ditto.
(TeX-error-description-help): Ditto.

* tex.el (nil): Handle the case of a non-available crm.el with a
`condition-case', instead of using the third argument of
`require', not recognized by XEmacs 21.4.
(and): Check whether dbus support is available before requiring
dbus.el.
Fix remaining XEmacs compilation issues.

* tex-buf.el (TeX-error-description-error): Do not use the
`default' display, not supported by GNU Emacs 21 and XEmacs 21.4.
(TeX-error-description-warning): Ditto.
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.
Fix other runtime issues in XEmacs and Emacs 21.

* tex-buf.el (TeX-next-error): Do not pass `reparse' argument to
`next-error' in XEmacs as it is not supported.
(TeX-error-overview-frame, TeX-error-overview-buffer-name): Move
before their first use in order to prevent a runtime error in
XEmacs and GNU Emacs 21.  Reported by Ikumi Keita.
(TeX-parse-TeX): Manually set `item' to nil when
`TeX-error-last-visited' is negative.
(TeX-error-description-error, TeX-error-description-warning)
(TeX-error-description-tex-said): Set to nil in XEmacs and GNU
Emacs 21.  Reported by Ikumi Keita.