Use LaTeX-biblatex-use-Biber to manually select bibliography processor.

* tex-style.el: Update coyright years.
(LaTeX-biblatex-use-Biber): New variable.  Mark as
safe-local-variable.

* latex.el (TeX-latex-mode): Add setting of `LaTeX-using-Biber' to
`TeX-update-style-hook'.

* style/biblatex.el ("biblatex"): Use `LaTeX-biblatex-use-Biber'
value to set `LaTeX-using-Biber' when it is set locally.

* tex-bar.el: Update copyright years.
(LaTeX-install-toolbar): Append toolbar refresh to
`TeX-update-style-hook' instead of prepending it.

* doc/auctex.texi:  Update copyright years.
(Selecting a Command): Document `LaTeX-biblatex-use-Biber'.

* doc/changes.texi: Mention `LaTeX-biblatex-use-Biber' change.