Use only rand(), not random().

We used both rand() and random() in our code. Since it returns an int, we have
to use %x in our format strings instead of %lx. This fixes a crash under
Windows when cross-compiling tinc with a recent version of MinGW.

src/net_packet.c

@@ -87,7 +87,7 @@ void send_mtu_probe(node_t *n)
 			return;
 		}
 
-		len = n->minmtu + 1 + random() % (n->maxmtu - n->minmtu);
+		len = n->minmtu + 1 + rand() % (n->maxmtu - n->minmtu);
 		if(len < 64)
 			len = 64;