Revert "Remove dmenumon variable"

This reverts commit c2b748e793.

Revert back this change. It seems to not be an edge-case anymore since
multiple users have asked about this new behaviour now.

dwm.c

@@ -1639,6 +1639,8 @@ 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));