net: tcp: Implement Keep-alive support

When a TCP connection is established, if there is no data exchange
between the two parties within the set time, the side that enables
TCP Keep-alive will send a TCP probe packet with the same sequence
number as the previous TCP packet. This TCP probe packet is an empty
ACK packet (the specification recommends that it should not contain
any data, but can also contain 1 nonsense byte, such as 0x00.). If
there is no response from the other side after several consecutive
probe packets are sent, it is determined that the tcp connection has
failed, and the connection is closed.

The keep-alive default parameters are aligned with Linux defaults.

Signed-off-by: Horse Ma <mawei@coltsmart.com>
Signed-off-by: Robert Lubos <robert.lubos@nordicsemi.no>