Implement SPNEGO flags in curb

Hello,

This patch adds another pass through feature for the setopt feature allowing to use SPNEGO authentication with credentials delegation *if selected by the user*. 

Default behavior remains entirely unchanged but with this patch it is possible to write code such as: 

    Proc.new do |curl|
      curl.http_auth_types = [ :gssnegotiate ]
      curl.setopt(Curl::CURLOPT_GSSAPI_DELEGATION, Curl::CURLGSSAPI_DELEGATION_FLAG)
    end

And have the credentials delegated as expected by libcurl.