Drop explicit support for F13-F20 and change to match the xterm terminfo
entry:

        F13-F24 are S-F1 to S-F12
        F25-F36 are C-F1 to C-F12
        F37-F48 are C-S-F1 to C-S-F12
        F49-F60 are M-F1 to M-F12
and     F61-F63 are M-S-F1 to M-S-F3

This should be no difference for applications inside tmux, but means
that any key binding for F13 will need to be replaced by S-F1 and so on.