If liblxc is used multi-threaded do_lxcapi_save_config() could be called from
threads that fork() which to not risk ending up with invalid locking states we
should avoid using functions like fopen() that internally allocate memory and
use locking. Let's replace it with the async-signal safe combination of
open() + write().
Signed-off-by: Christian Brauner <christian.brauner@ubuntu.com>