Improve defaults for filling.

* latex.el (LaTeX-fill-break-at-separators): Default to opening
and closing math switches.
Fix missing quote.
Squash a byte-compiler warning.

* latex.el (LaTeX-close-environment): Remove non-interactive
`next-line' usage compile warning.
Advice instead of fset + redefine.

* context.el (ConTeXt-add-environments): Advice instead of
renaming and redefining generated function.

* latex.el (LaTeX-add-bibliographies): Advice instead of
renaming and redefining generated function.
(LaTeX-add-environments): Ditto.
Fix bug causing endless package queries.

* latex.el (LaTeX-arg-usepackage-read-packages-with-options): Fix
bug that caused AUCTeX to query for packages infinitely.
Make TeX-completing-read-multiple return nil on empty input.

* multi-prompt.el (multi-prompt): Return nil on empty input.

* tex.el (TeX-completing-read-multiple): Define it so that empty
input results in nil across different emacs versions (<= 24.3
vs. later versions).

* style/biblatex.el ("biblatex"): Use
`TeX-completing-read-multiple' instead of
`completing-read-multiple'.
(LaTeX-arg-biblatex-cites): Use `TeX-completing-read-multiple' and
adapt handling of return value.

* style/pstricks.el (LaTeX-package-parameters): Adapt to
`TeX-completing-read-multiple' change.