Improve selection of the forward/inverse search method.

* tex.el (TeX-expand-list): Set
`TeX-source-correlate-output-page-function' when necessary, in the
"%(outpage)" expander.
(TeX-source-correlate-method): Add an alist as a possible value
and change the default.
(TeX-source-correlate-method-active): Convert to a function.
(TeX-source-correlate-expand-options): Use the
`TeX-source-correlate-method-active' function.
(TeX-source-correlate-mode): Remove setting of the
`TeX-source-correlate-output-page-function' variable and of the
now deleted `TeX-source-correlate-method-active' variable.
(TeX-source-specials-view-expand-options): Use the
`TeX-source-correlate-method-active' function.

* context.el: Update copyright years.
(ConTeXt-expand-options): Use the
`TeX-source-correlate-method-active' function.

* doc/auctex.texi (I/O Correlation): Update documentation of
`TeX-source-correlate-method'.

* doc/changes.texi: Mention change to the default value of
`TeX-source-correlate-method'.
Hide some entries of the Command menu when not available.

* tex.el (TeX-mode-specific-command-menu-entries): Hide "Previous Error"
and "Error Overview" entries when not available.