Improved Unix socket support

Added

• Add proxy_cacert, which completes the bindings for proxy SSL options. (#160, #344)
• Update bundled libcurl build configuration to enable support for Unix sockets automatically on Unix platforms. (#297, #309, #346)
• Add unix_socket_path that exposes more Unix socket control than the existing unix_socket method. (#347)
• Expose CURLOPT_CONNECT_TO via connect_to, which allows you to override the socket address that requests are made to. (#348)

Dependencies

• Bump bundled libcurl version to 7.71.1. (#343, #345)