The default appender for logs is currently the file one: this
make macros from log.h unusable as long as lxc_log_init() hasn't
been called. This prevents errors from even being printed on
stderr in lxc_caps_init() for example.

Let's make stderr the default appender.
Signed-off-by: Greg Kurz <gkurz@fr.ibm.com>
Signed-off-by: Daniel Lezcano <dlezcano@fr.ibm.com>