Based on further discussion on curl-library, I reverted yesterday's SOCKS5

Based on further discussion on curl-library, I reverted yesterday's SOCKS5
code to instead introduce support for a new proxy type called
CURLPROXY_SOCKS5_HOSTNAME that is used to send the host name to the proxy
instead of IP address and there's thus no longer any need for a new
curl_easy_setopt() option.

The default SOCKS5 proxy is again back to sending the IP address to the
proxy.  The new curl command line option for enabling sending host name to a
SOCKS5 proxy is now --socks5-hostname.