net: tcp: Issue connection callback on RST

A connection refused needs to be exposed to the user, otherwise the
connection will be stuck as a zombie forever.

This patch also adds a ENOTCONN check in net_context_recv() to match
the one that was already there in net_context_send().

Change-Id: I4f9ae46dd849f68ed97976add7da5daf1932cf55
Signed-off-by: Andy Ross <andrew.j.ross@intel.com>