valgrind drd tool shows conflicting stores happening at lxc_global_config_value@src...

valgrind drd tool shows conflicting stores happening at lxc_global_config_value@src/lxc/utils.c (v2)

Conflict occurs between following lines

[...]
269         if (values[i])
270                 return values[i];
[...]

and

[...]
309         /* could not find value, use default */
310         values[i] = (*ptr)[1];
[...]

fix it using a specific lock dedicated to that problem as Serge suggested.

Also introduce a new autoconf parameter (--enable-mutex-debugging) to convert mutexes to error reporting type and to provide a stacktrace when locking fails.

Signed-off-by: S.Çağlar Onur <caglar@10ur.org>
Signed-off-by: Serge Hallyn <serge.hallyn@ubuntu.com>