wifi: esp_at: cleanup sockets at network close

Some sockets (UDP sockets at least) do not generate "<N>,CLOSED"
messages when the WiFi network drops. As a result the networking stack
thinks these sockets are still open after the network has dropped, and
after any subsequent reconnections.

This affects the DNS resolver library in particular, which leaves UDP
sockets open permanently by default.

Manually close these sockets when the network drops to ensure a clean
state the next time the network connects.

Signed-off-by: Jordan Yates <jordan.yates@data61.csiro.au>