(gnus-compile): Modify the actual format specs as well; don't bind

(gnus-compile): Modify the actual format specs as well; don't bind
`gnus-tmp-func'.
(gnus-update-format-specifications): Use `gnus-update-format-specification-1'.
(gnus-update-format-specification-1): New function.
(gnus-format-specs-compiled): Modify the form.
(TopLevel): Require `alist'.
(gnus-compile): Don't modify the value of `gnus-format-specs', generate
compiled specs in `gnus-format-specs-compiled' instead; don't touch the
dribble buffer.
(gnus-format-specs-compiled): New internal variable.