[sr-dev] [kamailio/kamailio] tls: fix OpenSSL engine in child processes (#2840)

[aalba6675]

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

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