Cygwin: change virtual_ftype_t to not rely on negative values

So far negative values were denoting files, positive values
denoting directories.  We should prefer a less error prone
method.  Redefine virtual_ftype_t to contain only positive
values and replace checks for negativ or positive values with
inline functions virt_ftype_isfile() and virt_ftype_isdir().

Drop outdcated comments referring to numerical virtual_ftype_t
values.

Signed-off-by: Corinna Vinschen <corinna@vinschen.de>

winsup/cygwin/fhandler_virtual.cc

@@ -45,7 +45,7 @@ fhandler_virtual::opendir (int fd)
   DIR *res = NULL;
   size_t len;
 
-  if (exists () <= 0)
+  if (!virt_ftype_isdir (exists ()))
     set_errno (ENOTDIR);
   else if ((len = strlen (get_name ())) > PATH_MAX - 3)
     set_errno (ENAMETOOLONG);