Set up the split for `nnmail-split-fancy'.
Sets things up so that nnmail-split-fancy is used for mail
splitting, and defines the variable nnmail-split-fancy according with
group parameters.
If AUTO-UPDATE is non-nil (prefix argument accepted, if called
interactively), it makes sure nnmail-split-fancy is re-computed before
getting new mail, by adding `gnus-group-split-update' to
`nnmail-pre-get-new-mail-hook'.
A non-nil CATCH-ALL replaces the current value of
`gnus-group-split-default-catch-all-group'. This variable is only used
by gnus-group-split-update, and only when its CATCH-ALL argument is
nil. This argument may contain any fancy split, that will be added as
the last split in a `|' split produced by `gnus-group-split-fancy',
unless overridden by any group marked as a catch-all group. Typical
uses are as simple as the name of a default mail group, but more
elaborate fancy splits may also be useful to split mail that doesn't
match any of the group-specified splitting rules. See
`gnus-group-split-fancy' for details.
(fn &optional AUTO-UPDATE CATCH-ALL)