Simplify meaning of CONTAINER-URL; it no longer does anything special

Simplify meaning of CONTAINER-URL; it no longer does anything special
for IMAP URLs.  Define new operation CONTAINER-URL-FOR-PROMPT that has
the behavior that CONTAINER-URL used to.

Implement new operation URL-CHILD-NAME which is now the complement to
CONTAINER-URL.  These operations now return compatible halves of the
URL, which may be recombined using MAKE-CHILD-URL to obtain the
original URL.

Change implementation of URL-PRESENTATION-NAME; it now just calls
URL-CHILD-NAME, stripping a slash off the end if present.

Clarify definition of URL-BASE-NAME.

Revert CONTAINER-CONTENTS back to CONTAINER-URL-CONTENTS.  Maybe
tomorrow I'll change my mind again.