net: tcp: Signal EOF with a NULL buffer in the callback

TCP didn't actually have a way to signal synchronous receipt of a FIN
packet.  Extend the recv_cb API to allow a NULL buf argument with
status==0 (by analogy to Unix's zero-length read) to signal EOF.

Update docs too, and also echo_server which wasn't prepared to handle
this situation.

Change-Id: I7dc08f9e262a81dcad9c670c6471898889f0b05d
Signed-off-by: Andy Ross <andrew.j.ross@intel.com>
3 files changed