Replace completion-base-size by completion-base-position to fix bugs

Replace completion-base-size by completion-base-position to fix bugs
such as (bug#4699).
* simple.el (completion-base-position): New var.
(completion-base-size): Mark as obsolete.
(choose-completion): Make it work for mouse events as well.
Pass the new base-position to choose-completion-string.
(choose-completion-guess-base-position): New function, extracted from
choose-completion-delete-max-match.
(choose-completion-delete-max-match): Use it.  Make obsolete.
(choose-completion-string): Use the new base-position info.
(completion-root-regexp): Delete.
(completion-setup-function): Preserve completion-base-position.
Eliminate obsolete base-size manipulation.
* minibuffer.el (display-completion-list): Don't mess with base-size.
(minibuffer-completion-help): Set completion-base-position instead.
* mouse.el (mouse-choose-completion): Redefine as a mere alias to
choose-completion.
* textmodes/bibtex.el (bibtex-complete):
* emacs-lisp/crm.el (crm--choose-completion-string):
Adjust to new calling convention.
* complete.el (partial-completion-mode): Use minibufferp to avoid
bumping into incompatible change to choose-completion-string-functions.
* ido.el (ido-choose-completion-string): Make its calling convention
more permissive.
* comint.el (comint-dynamic-list-input-ring-select): Remove obsolete
base-size manipulation.
(comint-dynamic-list-input-ring): Use dotimes and push.
* iswitchb.el (iswitchb-completion-help): Remove dead-code call to
fundamental-mode.  Use `or'.