net: tcp2: Implement accept() and connect()

Register the TCP connection when accept() and connect() is called.
With the connection registration net_context will have the necessary
callback pointer set up, whereby net_context can call the proper
function when receiving packets for the TCP connecton.

With the new TCP stack this callback is always the same function.

Signed-off-by: Patrik Flykt <patrik.flykt@intel.com>
Signed-off-by: Oleg Zhurakivskyy <oleg.zhurakivskyy@intel.com>