Query CSI ? u at startup and push the disambiguate flag on the alternate
screen when the terminal answers; CSI u reports are mapped back to legacy
input (Shift/Ctrl+Enter insert a newline, Ctrl+Backspace deletes a word).
Ctrl/Alt+Left/Right move by word, Ctrl/Alt+Delete delete the next word.
Our define_key codes move above the range ncurses uses for terminfo
extended keys.