These functions provide a way to both start/initialize
  and stop/uninitialize the libcurl engine.
  There is no need to call
  curlGlobalInit as it is done implicitly the
  first time one uses the libcurl facilities.
  However, this function does permit one to explicitly
  control how it is initialized.
  Specifically, on Windows one might want to
  avoid re-initializing the Win32 socket facilities
  if the  host application (e.g. R) has already done this.
curlGlobalInit should only be called once per R session.
  Subsequent calls will have no effect, or may confuse the libcurl engine.
One can reclaim the resources the libcurl engine is
  consuming via the curlGlobalCleanup function
  when one no longer needs the libcurl facilities in
  an R session.