[tcp] Avoid setting PSH flag when SYN flag is set

Some firewall devices seem to regard SYN,PSH as an invalid flag
combination and reject the packet.  Fix by setting PSH only if SYN is
not set.

Reported-by: DSE Incorporated <dseinc@gmail.com>

src/net/tcp.c

@@ -471,6 +471,8 @@ static int tcp_xmit ( struct tcp_connection *tcp, int force_send ) {
 		tsopt->tsopt.tsval = ntohl ( currticks() );
 		tsopt->tsopt.tsecr = ntohl ( tcp->ts_recent );
 	}
+	if ( ! ( flags & TCP_SYN ) )
+		flags |= TCP_PSH;
 	tcphdr = iob_push ( iobuf, sizeof ( *tcphdr ) );
 	memset ( tcphdr, 0, sizeof ( *tcphdr ) );
 	tcphdr->src = tcp->local_port;
@@ -478,7 +480,7 @@ static int tcp_xmit ( struct tcp_connection *tcp, int force_send ) {
 	tcphdr->seq = htonl ( tcp->snd_seq );
 	tcphdr->ack = htonl ( tcp->rcv_ack );
 	tcphdr->hlen = ( ( payload - iobuf->data ) << 2 );
-	tcphdr->flags = ( flags | TCP_PSH );
+	tcphdr->flags = flags;
 	tcphdr->win = htons ( tcp->rcv_win );
 	tcphdr->csum = tcpip_chksum ( iobuf->data, iob_len ( iobuf ) );