Problem: Since 48e2a736, prompt messages are handled by an actual
active cmdline, resulting in `State` no longer being equal
to `MODE_CONFIRM` which is used in some places. E.g. to
specify the current `mode()` or to re-emit a confirm message.
Solution: Replace `MODE_CONFIRM` with a new `MODE_CMDLINE` sub-mode when
`ccline.one_key/mouse_used` is set. Use it to avoid clearing
mouse_used prompt messages, and to re-emit one_key messages
(when ext_messages is inactive, for which this is unnecessary).
(cherry picked from commit e876a739ee)
89959ab9dc