some fixes: calloc was wrong; catch traps, and let divides

by 0 generate a signal, if on a unix machine

lang/cem/libcc/gen/rename.c

@@ -0,0 +1,9 @@
+int
+rename(from, to)
+	char *from, *to;
+{
+	(void) unlink(to);
+	if (link(from, to) < 0) return -1;
+	(void) unlink(from);
+	return 0;
+}