tls_init.c calls OPENSSL_init_ssl(); this initializes the global engine linked-list and this cannot be reset in the child.

To avoid linked-list corruption we manually instantiate the engine object required for loading private keys instead of relying on CONF_modules_load_file().

Updates to doc/.

Addresses #2839

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#### Description The call to `OPENSSL_init_ssl()` in `tls_init.c` results in the creation of the engine linked-list in the parent.

This affects per-child engine private keys as there is no api to reinitialize the engine linked-list in the child.

This PR removes the call to `CONF_modules_load_file()` which causes linked-list corruption and replaces the initialization of engine private keys in the child with with other api calls which do not manipulate global objects.

You can view, comment on, or merge this pull request online at:

https://github.com/kamailio/kamailio/pull/2840

-- Commit Summary --

* tls: fix OpenSSL engine in child processes

-- File Changes --

M src/modules/tls/doc/hsm_howto.xml (15) M src/modules/tls/tls_mod.c (93)

-- Patch Links --

https://github.com/kamailio/kamailio/pull/2840.patch https://github.com/kamailio/kamailio/pull/2840.diff