net: sockets: Make use of the status field reported by TCP

Make use of the status field, reported by TCP, in the socket receive
callback. This allows to differentiate a graceful connection shutdown
from actual errors at TCP level (transmission timeout or RST received).
In case of error reported from TCP layer, set a new SOCK_ERROR flag on
the socket, and store the error code in the net_context user_data.

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