When Kitty keyboard protocol is not available (e.g. inside tmux), fall
back to xterm modifyOtherKeys mode 2 so that Shift+Enter, Ctrl+Enter,
and other modified keys are distinguishable from plain Enter.
tmux users need to add to ~/.tmux.conf:
set -g extended-keys on
set -g extended-keys-format csi-u
closes#1872
