net: context: Bind default address for UDP

This patch makes net_context_sendto() work independently without calling
net_context_connect() first. It will bind default address and port if
necessary.

Also, since receive callback should be provided before sending data in
order to receive the response, bind default address and port to prevent
providing an unbound address and port to net_conn_register().

Signed-off-by: Aska Wu <aska.wu@linaro.org>