Cleanup: move libcurl specifc code into curl.c
This cleans the libcurl usage up a bit. It moves common code used to interact with libcurl into a single new file curl.[ch]. Notes: * It does not alter any features nor fixes any bugs. * Thew REVIEW about strdup(self->url) in event_url.c as been processed. According to the manpage for curl_easy_setopt()/CURLOPT_URL there is no need for us to copy. * URL Auth as well as URL triggers have sent ICECAST_VERSION_STRING as User-Agent:, this has be corrected to what is set in the config file. * As curl.c is now a single point for setting parameters all the TLS parameters should be reviewed and set (based on config). Please test!