New variable to pass options to the TeX processor.

* tex.el (TeX-command-extra-options): New customizable variable.
(TeX-expand-list): New `%(extraopts)' expander.
(TeX-command-list): Use `%(extraopts)'.

* doc/auctex.texi (Processor Options): Document
`TeX-command-extra-options'.

* doc/changes.texi: Document it.
Prompt for packages inside LaTeX-env-document.

* latex.el (LaTeX-arg-usepackage-read-packages-with-options): New
function.  Now options are requested only if at least one package
has been provided.
(LaTeX-arg-usepackage-insert): New function.
(LaTeX-arg-usepackage): Use
`LaTeX-arg-usepackage-read-packages-with-options' and
`LaTeX-arg-usepackage-insert'.
(LaTeX-insert-usepackages): New function.
(LaTeX-env-document): Use it.

* doc/auctex.texi (Environments): Document new behavior of
`LaTeX-env-document'.

* doc/changes.texi: Mention it.
Fix selection of default environment in LaTeX-environment.

* latex.el (LaTeX-environment): Do not set
`LaTeX-default-environment' to `environment' if the latter is
equal to the current default environment.