Implement the logging callback

[zimmerle]

One of the ModSecurity logging features depending on write content on the webserver logging. In order to write in the webserver log, we need to create an wrapper/callback inside this apache module, that wrapper will be called by libModSecurity, presenting some content to be saved.

The callback used with the nginx implementation is available here:
https://github.com/SpiderLabs/ModSecurity-nginx/blob/master/src/ngx_http_modsecurity_log.c#L25-L32

[csanders-git]

Log callback is specified via msc_set_log_cb(conf->modsec, ngx_http_modsecurity_log);