Fix `TeX-next-error'.

* tex-buf.el (TeX-help-error): Let-bind `inhibit-read-only' when
updating the *TeX Help* buffer.
Fix compile error with emacs bzr which assumes elisp files are UTF-8 encoded.

* tex-jp.el: Add `coding: iso-2022-7bit-unix' file local variable.
Fix coding once again.

* tex-jp.el: Change to `coding: iso-2022-jp-unix'.
Put coding cookie in elisp header line.
Simplify TeX-doc-backend-alist.

* tex.el (TeX-doc-backend-alist): Remove unnecessary `info' check.
Better check for DBUS support.

* tex.el (TeX-evince-dbus-p): Improved DBUS availability check.