[sr-dev] [kamailio/kamailio] tls: add support for OpenSSL engine and private keys in HSM (#1484)

[aalba6675]

Thanks for the comments, I summarize actionable items at the bottom as the conversation develops. I can push further commits, and do the final squash when it can be accepted.

1. Preprocessor defines `OPENSSL_NO_ENGINE` - followed nginx and HAProxy where they use this to omit compile-time code that uses OpenSSL `ENGINE_xxxx` functions. Frankly I doubt any modern OpenSSL actually defines this. Same purpose as `OPENSSL_NO_ECDH` in existing `tls.c`.

    At runtime it might be difficult as the symbol won't be in the users `libcrypto.so`. If we include these symbols, then the users `libcrypto.so` is required to have engine support (even if they don't use it)

    Currently the runtime use is controlled by the proposed modparam `engine`, but ENGINE symbols are still UND in `tls.so`.

1.  whitespace - added to TODO list below: it was a code editor setting, my bad
1. documentation - added to TODO list

Sample nginx code (because of `ENGINE_*` symbols). HAProxy has similar constructs:
```
#ifndef OPENSSL_NO_ENGINE
        u_char      *p, *last;
        ENGINE      *engine;
        EVP_PKEY    *pkey;
        p = key->data + sizeof("engine:") - 1;
        last = (u_char *) ngx_strchr(p, ':');
        if (last == NULL) {
            ngx_conf_log_error(NGX_LOG_EMERG, cf, 0,
                               "invalid syntax in \"%V\"", key);
            return NGX_ERROR;
        }
        *last = '\0';
        engine = ENGINE_by_id((char *) p);
```

TODO list:
* revert code editor gratuitous whitespace changes
* documentation updates for new configuration directives

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