net: tcp: Use correct seqnum for tcp_out and tcp_out_ext

Based on TCP Spec., the outgoing TCP packets shoud use SND.NXT as
the seqnum. In Zephyr, the conn->seq works as the SND.UNA and the
conn->seq + conn->unacked_len works as the SND.NXT. Currently, it
uses SND.UNA in tcp_out() as the seqnum, which might get dropped
as old packets and could not deliver the message to the peer.
A few exceptions use SND.NXT - 1 as the seqnum are: keepalive,
zero-window-probe, FIN/SYN retransmissions. And, for closing a
connection, Zephyr won't send out FIN until all the data has been
ACKed, so the conn->unacked_len is 0 and it is ok to use conn->seq
as the SND.NXT.

Signed-off-by: Shrek Wang <inet_eman@outlook.com>