kf* terminfo capabilities are poorly defined and rxvt uses them in a

different way from xterm, so add a feature flag for rxvt to make tmux
ignore the capabilities and instead rely on its builtin definitions.

tmux.1

@@ -3656,6 +3656,12 @@ Allows setting the cursor style.
 Supports extended keys.
 .It focus
 Supports focus reporting.
+.It ignorefkeys
+Ignore function keys from
+.Xr terminfo 5
+and use the
+.Nm
+internal set only.
 .It margins
 Supports DECSLRM margins.
 .It mouse