Computes nnmail-split-fancy from group params and CATCH-ALL.
It does this by calling by calling (gnus-group-split-fancy nil
nil CATCH-ALL).
If CATCH-ALL is nil, `gnus-group-split-default-catch-all-group' is used
instead. This variable is set by `gnus-group-split-setup'.
(fn &optional CATCH-ALL)