net: tcp: Add Kconfig option for auto-accepting clients

Add CONFIG_NET_TCP_AUTO_ACCEPT option which can be used to
automatically accept incoming data connection even if the
application has not yet called accept(). This can speed up
data transfer from peer to the application.
Problem with this is that if the peer sends lot of data and
we have limited amount of net_buf's available, then we can
run out of them which is very bad situation and can lead to
deadlocks. Because of this, the setting is turned off by default.

Signed-off-by: Jukka Rissanen <jukka.rissanen@linux.intel.com>