Use smaller, stable, int for history index

The size of size_t depends on the architecture, so we need to have
different conversion to and from strings. But we don't really need that
range, so avoid the issue by using a standard integer size.

vncviewer/parameters.cxx

   if (res != ERROR_SUCCESS)
     throw rdr::SystemException(_("Failed to create registry key"), res);
 
-  size_t index = 0;
+  unsigned index = 0;
   assert(SERVER_HISTORY_SIZE < 100);
   char indexString[3];
 
     throw rdr::SystemException(_("Failed to open registry key"), res);
   }
 
-  size_t index;
+  unsigned index;
   const DWORD buffersize = 256;
   char indexString[3];