include comment on windows services
harmonize asprintf with OpenSSH

 * use the original name for the file from OpenSSH (remove duplicate version)
 * add va_copy/__va_copy checks to configure
 * incorporate proposed fixes to openssh version:
   + include more system headers directly for various definitions
   + limit the scope of va_copy/va_end to their affected vsnprintf calls
   + simplify error handling, removing a dead assignment