Switch the prompt code to return an empty string when the user enters no
response and reserve NULL for an explicit cancel. Change all callbacks to treat
them the same so no functional change.
Also add cancel key bindings to emacs mode which were missing.
vi(1)-style half page scroll in copy and scroll modes. Move the vi full page
scroll key to C-b instead of C-u and use C-u/C-d for half page scrolling with
vi keys. In emacs mode, half page scrolling is bound to M-Up and M-Down.
Suggested by merdely (about a year ago :-)).
Next step towards customisable mode keys: build each default table of keys
into a named tree on start and use that for lookups. Also add command to string
translation tables and modify list-keys to show the mode key bindings (new
-t argument).
Change mode key bindings from big switches into a set of tables. Rather than
lumping them all together, split editing keys from those used in choice/more
mode and those for copy/scroll mode.
Tidier and clearer, and the first step towards customisable mode keys.
Detect backspace by looking at termios VERASE and translate it into \177
(which matches screen's behaviour if not its termcap/terminfo entry). The
terminfo kbs cap is often wrong or missing so it can't be used, and just
assuming \177 may be wrong.
