Catch up with the renaming of the "union" filesystem to "unionfs".

Fixes a problem where directory entries could show up twice: once
on the top layer of the union stack, and once on the bottom layer.

Approved by:	re (rwatson)

lib/libc/gen/opendir.c

@@ -118,7 +118,7 @@ __opendir2(name, flags)
 
 		if (_fstatfs(fd, &sfb) < 0)
 			goto fail;
-		unionstack = !strcmp(sfb.f_fstypename, "union")
+		unionstack = !strcmp(sfb.f_fstypename, "unionfs")
 		    || (sfb.f_flags & MNT_UNION);
 	} else {
 		unionstack = 0;