(completion-boundaries): New function.

(completion-boundaries): New function.
(completion--some): Delay errors.
(complete-with-action, completion-table-with-context): Handle `boundaries'.
(completion--try-word-completion): Avoid partial-completion
when the user hasn't entered anything yet.
(minibuffer-local-map, minibuffer-local-filename-completion-map)
(minibuffer-local-must-match-map, minibuffer-local-completion-map)
(minibuffer-local-must-match-filename-map, minibuffer-local-ns-map):
Setup default keybindings.
(completion--embedded-envvar-re): New var.
(completion--embedded-envvar-table): Use it.  Handle `boundaries' case.
(completion--file-name-table): Handle `boundaries' case.
(completion-pcm--pattern->regex): Avoid pathological backtracking.
(completion-pcm--all-completions): Add a `prefix' arg.
(completion-pcm--find-all-completions): New function.
(completion-pcm-all-completions, completion-pcm-try-completion): Use it.