Add pthread_atfork check to configure.ac and uses it when necessary,
Introduces tls.m4 macro for checking thread-local storage support, Puts
values array into thread-local storage
(lxc_global_config_value@src/lxc/utils.c), Removes
static_lock/static_unlock from LXC code.

Lastly, it introduces a warning for bionic users about multithreaded
usage of LXC.

(requires 64b1be29 to be reverted first)
Signed-off-by: S.Çağlar Onur <caglar@10ur.org>
Acked-by: Stéphane Graber <stgraber@ubuntu.com>