Similar to gatchat, introduce read_watcher_destroy_notify and do
not trigger user disconnect at g_at_shutdown. Delay destroy of
gatserver until read_watcher is destroyed.
According to V.250 spec section 5.2.4 Repeating a command line, if
the prefix "A/" or "a/" is received, we should execute the last
command immediately. No need to meet \r.
It's the basic skeleton of GAtServer, including new/shutdown, ref/
unref, received_data/parse_buffer and set_discuss/set_debug.
GAtServer is to emulate the server side of AT conversation. It
complies with V.250 and 27.007 spec to accept AT command like
ATV1, ATE0 and extended command like AT+CLCC. Upper layer could
create customize server to expose TTY, tcp or unix socket to
client side application.