access: `*` for any one key in a capability's path

A capability's path covered by prefix only, so what is the same at every key of a map
(each node's log filter, say, at `_continuum/local/<id>/log` from the next patch on)
took a capability per key. Now a `*` segment stands for any one key, as it already does
in signed paths and sealed places (`envelope`, `sealed`, which now share one `agree`):

* covering (`Capability::covers`): the pattern no longer than the path, and agreeing
  with it segment by segment;
* listings (`reaches`): pattern and path agreeing on the segments both have, so every
  key on the way shows, nothing past where the pattern differs;
* narrowing (`Capability::holds`, for a token's limit narrowed again and a refresh's):
  at or below, a `*` only under a `*`: a limit naming one key never widens to all.

A `*` first is outside protocol data, so it never covers `_continuum`, as `[]` doesn't.
A key that is `*` itself can't be named alone. Every node judges changes by
capabilities, so all must read them alike: they run the same version.

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