For consistency, as we seem to have a bit of a mixed bag, changed all
instances of ipv4 and ipv6 in comments and documentations to use the
correct case.
Blocking connect on the socket has been removed from opensocket
callback. opensocket just opens a new socket and gives it back to
libcurl and libcurl will take care of the connect. sockopt_callback has
also been removed, as it is no longer required.