Alexey Pesternikov introduced CURLOPT_OPENSOCKETFUNCTION and

CURLOPT_OPENSOCKETDATA to set a callback that allows an application to replace
the socket() call used by libcurl. It basically allows the app to change
address, protocol or whatever of the socket. (I also did some whitespace
indent/cleanups in lib/url.c which kind of hides some of these changes, sorry
for mixing those in.)