(sh-shell-variables-initialized): New variable.

(sh-shell-variables-initialized): New variable.
(sh-mode): Make sh-shell-variables and
sh-shell-variables-initialized local.
(sh-set-shell): Don't scan the buffer for variable assignments.
Set sh-shell-variables-initialized to nil.
(sh-shell-initialize-variables): New function.
(sh-add-completer): New function.  Initialize sh-shell-variables
here if not yet done.
(sh-add): Use sh-add-completer for completion.