(gnus-compile): Don't modify the value of `gnus-format-specs', generate

(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-update-format-specifications): Bind `gnus-format-specs' to
`gnus-format-specs-compiled' if the latter is non-nil;
use `gnus-update-format-specifications-1'.
(gnus-update-format-specifications-1): Rename from
`gnus-update-format-specifications';
update the value of `gnus-newsrc-file-version' if the updating is forced.
(gnus-format-specs-compiled): New internal variable.