[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

 		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;
 	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 ) );