Really fix bug in biblatex style file.

* style/biblatex.el ("biblatex"): Do not quote at all
`TeX-arg-key-val' arguments.
Run style files of the bibliography files when inserting bibliography macros.

* latex.el (TeX-arg-bibliography): Run style files associated to
the bibliography database files.

* style/biblatex.el (LaTeX-arg-addbibresource): Run style file
associated to the bibliography database file.
Document missing folding variables.

* doc/auctex.texi (Folding): Document `TeX-fold-auto' and
`TeX-fold-unfold-around-mark'.