Change again method to determine whether a label should be inserted.

* latex.el (LaTeX-insert-label): Remove.
(LaTeX-label): Add a new mandatory argument.  Do not use
`LaTeX-insert-label'.  Determine the prefix at the beginning of
the function and insert the label only if the prefix is non nil.
(LaTeX-section-label, LaTeX-env-figure, LaTeX-env-label): Use the
second mandatory argument of `LaTeX-label'.

* style/amsmath.el: Update copyright years.
("amsmath"): Append the environments to `LaTeX-label-alist'
instead of prepeding them.  Use the second mandatory argument of
the `LaTeX-label' function.

* style/longtable.el: Update copyright years.
("longtable"): Move addition of "longtable" environment to
`LaTeX-label-alist' inside the style hook.  Append the environment
to the alist instead of prepending it.  Use the second mandatory
argument of the `LaTeX-label' function.

* doc/auctex.texi (Sectioning, Environments): Remove references to
`LaTeX-insert-label'.
(Environments): Document `LaTeX-label-alist'.

* doc/changes.texi: Remove references to `LaTeX-insert-label'.