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_procsysvipc.cc

@@ -52,9 +52,6 @@ static const virt_tab_t procsysvipc_tab[] =
 static const int PROCSYSVIPC_LINK_COUNT =
   (sizeof (procsysvipc_tab) / sizeof (virt_tab_t)) - 1;
 
-/* Returns 0 if path doesn't exist, >0 if path is a directory,
- * -1 if path is a file.
- */
 virtual_ftype_t
 fhandler_procsysvipc::exists ()
 {