Working towards allowing multiple Maildir prefixes.

  This commit gives us the ability to get/set a table against
 a string-variable.

  This updates #202.

  The next step is to make the use of the `maildir_prefix` primitive
 use the multi-version of the multi-variable storing code, if it is given
 a table.

  (Or just do that globally, even if there is one member.  But that
 will require documentation changes, so it is probably best avoided.)