Use TeX-esc as completion separator for units in siunitx.el.

* style/siunitx.el: Update copyright years and specify last
`siunitx' version supported.  Hard wrap lines longer than 80
columns.
(LaTeX-siunitx-regexp): Move the escape character out of the group
matching the unit name.
(LaTeX-arg-siunitx-unit): Add `prefix' argument.  Replace the
space with `TeX-esc' as the completion separator.
(LaTeX-arg-define-siunitx-unit): Define a default prompt; set
`initial-input' to nil; use `TeX-esc' as prefix to the given
input.
("siunitx"): Remove the escape character from the unit names.