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!
