Be more specific in the DSR we are looking for so it doesn't get

confused with mouse sequences. Also set a flag and don't bother checking
for it if we have already seen it (same for DA), and don't check if we
never asked for it.

tmux.h

@@ -1227,6 +1227,8 @@ struct tty {
 #define TTY_OPENED 0x20
 #define TTY_FOCUS 0x40
 #define TTY_BLOCK 0x80
+#define TTY_HAVEDA 0x100
+#define TTY_HAVEDSR 0x200
 	int		 flags;
 
 	struct tty_term	*term;