LIBCURL-STRUCTS: new document

This is the first version of this new document, detailing the seven
perhaps most important internal structs in libcurl source code:

  1.1 SessionHandle
  1.2 connectdata
  1.3 Curl_multi
  1.4 Curl_handler
  1.5 conncache
  1.6 Curl_share
  1.7 CookieInfo