(ibuffer-default-display-maybe-show-predicates): New customizable

(ibuffer-default-display-maybe-show-predicates): New customizable
variable; whether to display buffers that match
`ibuffer-maybe-show-predicates' by default.
(ibuffer-maybe-show-predicates): Mention it in docstring.
(ibuffer-display-maybe-show-predicates): New variable.
(ibuffer-update): Prefix arg now toggles whether buffers that match
`ibuffer-maybe-show-predicates' should be displayed.
(ibuffer-mode): Set `ibuffer-display-maybe-show-predicates' locally to heed
`ibuffer-default-display-maybe-show-predicates'.
(ibuffer-redisplay-engine): Rename optional second arg to `ignore'.