Fixes to TeX-arg-index-tag, TeX-arg-cite, TeX-parse-arguments.

* latex.el: Update copyright years, there have been non-trivial
changes in 1998, 2001, 2002.
(TeX-arg-index-tag): Use `TeX-argument-prompt'.
(TeX-arg-cite): Ditto.

* tex.el: Update copyright years, there have been non-trivial
changes in 1995, 1998.
(TeX-parse-arguments): Move skipping of optional
arguments inside loop over all arguments, because optional
arguments may not be the first ones.  Remove unused let-bound
variable `skip-opt'.
(TeX-arg-literal): Remove FIXME comment, `optional' is the first
argument passed to all functions by `TeX-parse-argument'.