Lastlog searching with /

Add support for searches with / and ?.

Search begins with /<pattern> or ?<pattern> from command mode.

Internally, runs a /lastlog <pattern> <$max_matches>, and consumes the result (not printed).

Parse timestamps or do other cunning matching things to find the position of each match in the scrollback.

Jump the window-item view back to the most recent match with /sb goto ...

Each subsequent / (or n) in command-mode moves to the next match in the buffer. ? moves in the opposite direction.

Any new pattern invalidates the current jump list.

TODO

Need a way to cancel and jump back to /sb end
Finding the actual position of the matches might be tricky (especially without timestamps, or non-standard formats)
- Could compare entire lines by walking the window history, but that sounds slow.
Is it possible to highlight the matched pattern onscreen? Probably not easily, without remove/readding the line entry, or using gui_print (which is transient)