net: tcp: Send RST reply for unexpected TCP packets

Send RST as a reply for unexpected TCP packets in the following
scenarios:
1) Unexpected ACK value received during handshake (connection still open
   on the peer side),
2) Unexpected data packet on a listening port (accepted connection
   closed),
3) SYN received on a closed port.

This allows the other end to detect that the connection is no longer
valid (for example due to reboot) and release the resources.

Signed-off-by: Robert Lubos <robert.lubos@nordicsemi.no>