Add history variables in some style files.

* style/acro.el (LaTeX-acro-acronym-history): New variable.
(LaTeX-arg-acro-acronym): Use it.

* style/acronym.el (LaTeX-acronym-acronym-history): New variable.
(LaTeX-arg-acronym-acronym): Use it.

* style/beamer.el (LaTeX-beamer-frametitle-history): New variable.
("beamer"): Use it.
(TeX-arg-beamer-frametitle): Ditto.

* style/siunitx.el (LaTeX-siunitx-unit-history): New variable.
(LaTeX-arg-siunitx-unit): Use it.