net: tcp: Remove NULL-pkt support for tcp_in()

The NULL-pkt parameter for tcp_in() was designed for generating
a SYN packet to start the 1st TCP handshake. It is only used
in net_tcp_connect() and tp_input().
To simplify the tcp_in() code logic and make it better under-
standable, a tcp_start_handshake() is added for net_tcp_connect()
and tp_input() to use. Thus, the tcp_in() only handles the in-
coming TCP packets.

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