GSocket: fix GIOCondition on timed-out socket operation

GSocket: fix GIOCondition on timed-out socket operation

The docs for g_socket_set_timeout() claimed that if an async operation
timed out, the GIOCondition passed to the source callback would be
G_IO_IN or G_IO_OUT (thus prompting the caller to call
g_socket_receive/send and get a G_IO_ERROR_TIMED_OUT), but in fact it
ended up being 0, and gio/tests/socket.c was erroneously testing for
that instead of the correct value. Fix this.