Introduce OPENSSL_ATEXIT_CLEANUP env. variable.

Introduce OPENSSL_ATEXIT_CLEANUP env. variable.

libcrypto does not arm OPENSSL_cleanup() function as atexit(3) handler by default.
If application/user wants libcrypto to install OPENSSL_cleanup() as atexit handler,
then OPENSSL_ATEXIT_CLEANUP env. variable must be set.

If platform's libc does not provide atexit(3), then OPENSSL_ATEXIT_CLEANUP has no effect.

The OPENSSL_atexit() is wrapper of atexit(3) provided by libc now.

Reviewed-by: Neil Horman <nhorman@openssl.org>
Reviewed-by: Matt Caswell <matt@openssl.org>
(Merged from https://github.com/openssl/openssl/pull/29385)