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.