* lisp/completion.el: Use post-self-insert-hook.

* lisp/completion.el: Use post-self-insert-hook.

Fixes: debbugs:19400
(completion-separator-self-insert-command)
(completion-separator-self-insert-autofilling): Remove.
(completion-separator-chars): New var.
(completion-c-mode-hook, completion-setup-fortran-mode): Use it instead
of changing the keymap.
(completion--post-self-insert): New function.
(dynamic-completion-mode): Use it instead of rebinding keys.
(cmpl--completion-string): Rename from completion-string.
(add-completion-to-head, delete-completion): Let-bind it explicitly.