Revert "server: don't use SO_REUSEADDR socket option"

Without SO_REUSEADDR re-binding to addresses can fail with e.g.:
knotd: error: cannot bind address ::@53 TCP (address already in use)

This reverts commit 43c4c513.

src/contrib/net.c

@@ -166,8 +166,6 @@ static void unlink_unix_socket(const struct sockaddr_storage *addr)
 
 int net_bound_socket(int type, const struct sockaddr_storage *addr, enum net_flags flags)
 {
-	int ret;
-
 	/* Create socket. */
 	int sock = net_unbound_socket(type, addr);
 	if (sock < 0) {
@@ -179,6 +177,13 @@ int net_bound_socket(int type, const struct sockaddr_storage *addr, enum net_fla
 		unlink_unix_socket(addr);
 	}
 
+	/* Reuse old address if taken. */
+	int ret = sockopt_enable(sock, SOL_SOCKET, SO_REUSEADDR);
+	if (ret != KNOT_EOK) {
+		close(sock);
+		return ret;
+	}
+
 	/* Don't bind IPv4 for IPv6 any address. */
 	if (addr->ss_family == AF_INET6) {
 		ret = sockopt_enable(sock, IPPROTO_IPV6, IPV6_V6ONLY);