* lisp/minibuffer.el (complete-with-action): Return nil for the metadata and

* lisp/minibuffer.el (complete-with-action): Return nil for the metadata and
boundaries of non-functional tables.
(completion-table-dynamic): Return nil for the metadata.
(completion-table-with-terminator): Add default case, using
complete-with-action.
(completion--metadata): New function.
(completion-all-sorted-completions, minibuffer-completion-help): Use it
to try and avoid pathological performance problems.
(completion--embedded-envvar-table): Return `category' metadata.