Add new TeX-file-line-error option

This new option allows the user to select the style TeX prints error
messages with (also see FAQ number 8).

* tex.el (TeX-file-line-error): New customizable option.
  (TeX-expand-list): New expander "$(file-line-error)".
  (TeX-command-list): Use it.
  (LaTeX-command-style): Use it.

* doc/auctex.texi (Processor Options): Document `TeX-file-line-error'.

* doc/changes.texi: Mention `TeX-file-line-error'.