(completion-try-completion): Add `point' argument. Change return value.

(completion-try-completion): Add `point' argument.  Change return value.
(completion-all-completions): Add `point' argument.
(minibuffer-completion-help): Pass the new `point' argument.
(completion--do-completion): Pass the whole field to try-completion.
(completion--try-word-completion): Rewrite, making fewer assumptions.
(completion-emacs21-try-completion, completion-emacs21-all-completions)
(completion-emacs22-try-completion, completion-emacs22-all-completions)
(completion-basic-try-completion, completion-basic-all-completions): New funs.
(completion-styles-alist): Use them.