applied improved version of Martti Kühne's dmenu/multi monitor approach from dwm, no dmenuspawn required

dwm.c

@@ -1569,6 +1569,8 @@ sigchld(int unused) {
 
 void
 spawn(const Arg *arg) {
+	if(arg->v == dmenucmd)
+		dmenumon[0] = '0' + selmon->num;
 	if(fork() == 0) {
 		if(dpy)
 			close(ConnectionNumber(dpy));