* lisp/progmodes/sh-script.el: Provide simpl(e|istic) completion.

* lisp/progmodes/sh-script.el: Provide simpl(e|istic) completion.
(explicit-shell-file-name): Declare.
(sh--vars-before-point, sh--cmd-completion-table): New functions.
(sh-completion-at-point-function): New function.
(sh-mode): Use it.
(sh-smie--keyword-p): Remove unused argument.
(sh-smie-sh-backward-token, sh-smie-rc-backward-token): Remove unused
vars.
(sh-set-shell): Always setup SMIE, even if we use the
old indentation code.