* Implement container URLs, which are separate from folder URLs.

* Implement container URLs, which are separate from folder URLs.
  These are implemented as directories for file-based folders.  IMAP
  folders are also containers.

* Eliminate URL-CONTAINER-STRING; now there is URL-CONTAINER that
  returns a container URL.

* Rename URL-SELECTABLE? to URL-IS-SELECTABLE?.

* Eliminate "rmail" and "umail" protocols in favor of "file".  This
  now covers both Rmail and unix-mail folders, as well as directories.
  The actual file type is determined by probing the first few bytes of
  the file for known patterns.  The names "rmail" and "umail" are now
  treated as equivalent to "file" for upwards compatibility.

* Change prompting code in front end so that it is possible to specify
  that the returned folder satisfies URL-IS-SELECTABLE?.  Also add a
  procedure to prompt for a container.