Revert "Revert "Use pthread_atfork() to unlock mutexes after fork()""

This reverts commit 64b1be2903078ef9e9ba3ffcbc30a4dc9bc5cc6c.

Reverting in preparation for another implementation which is
bionic-compatible.

Signed-off-by: Stéphane Graber <stgraber@ubuntu.com>
check pthread_atfork and thread-local storage

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 64b1be2903078ef9e9ba3ffcbc30a4dc9bc5cc6c to be reverted first)

Signed-off-by: S.Çağlar Onur <caglar@10ur.org>
Acked-by: Stéphane Graber <stgraber@ubuntu.com>