Beginnings of work for search field feature of new pager
Refactor reader to work on selectable 'editable_line_t' so that most
commands can operate on pager search field
Eliminate dangerous command_length() function. Use editable_line_t in a
few places we missed.
Increased support for completion search field. Use btab (shift-tab) to
complete-and-search.
Support escape or up-arrow to cancel the completion search field.
Make arrow keys navigate completions instead of manipulate completion
search field
Make escape clear the current completion
Make return not execute the selected completion in the pager, instead
just accept it
Make COMPLETE_AND_SEARCH always show the search field. Also end paging upon
modifying the command line contents.