(gnus-namazu/group-prefixed-name): Return

(gnus-namazu/group-prefixed-name): Return
the whole name from a given group and a method, only when it
exists.  Otherwise, return nil.
(gnus-namazu/possible-real-groups): Removed.
(gnus-namazu/decode-group-name): The successor of the above
function.
(gnus-namazu/real-group-name): Use the above functions, and accept
the third optional argument, `method'.
(gnus-namazu/search): Change the order getting groups and article
numbers from file names.