[sr-dev] backporting atexit patches to branch 5.4

[Daniel-Constantin Mierla]

Hello,

I am planning to backport a series of patches from master to branch 5.4
to get kamailio working on newer linux distros (like Ubuntu 20.04) that
include recent versions of libssl/libcrypto. I got access to an Ununtu
20.04 deployments and I hope I tackled all the variants that can appear,
with forking, daemonizing in systemd-style or not-forking.

Apparently the libs uses the atexit callbacks for cleanup, but kamailio
exits after destroying shared memory, resulting in access to invalid
memory addresses. I could not sort out why is done at shutdown, because
kamailio is also cleaning the libssl context, but looks like something
in libcrypto is still not destroyed by libssl shutdown. On the other
hand, same issue seems exposed when tls module is not used, but there
are other modules linking to libssl directly or via intermediate library
(like http_client -> libcurl ->libssl).

Overall, there should be no change on runtime code, only initialization
exit cases (e.g., cfg error case or daemonizing with close of initial
parent) and shutdown. However, there is a new cli parameter --atexit
that is going to appear in 5.4, which is necessary because older linux
distros (e.g., Debain 10) do not expose the problem. The default is the
current behaviour, but in the future we may have to switch to the new mode.

Without the backporting, kamailio cannot be used (cannot be started) on
newer linux distros.

Cheers,
Daniel

-- 
Daniel-Constantin Mierla -- www.asipto.com
www.twitter.com/miconda -- www.linkedin.com/in/miconda
Funding: https://www.paypal.me/dcmierla