Clear urgency hint on sel == c

 - Run "tabbed xterm -into".
 - Open a new tab using Ctrl+Shift+Enter, switch to it using Ctrl+Tab.
 - Run "sleep 2; printf '\a'".
 - Quickly change back to the first tab (c = 0) and wait for the second
   one (c = 1) to get the urgency hint.
 - Close the first tab using Ctrl+Q or Ctrl+D. This sets sel = 0 and c =
   0, so focus() won't touch the urgency hint.
 - Now, the urgency hint on the second tab has not been cleared. Open a
   new tab to verify it.