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_netdrive.cc

@@ -145,8 +145,6 @@ create_thread_and_wait (int what, PVOID in, PVOID out, DWORD outsize,
   return ndi.ret;
 }
 
-/* Returns 0 if path doesn't exist, >0 if path is a directory,
-   -1 if path is a file, -2 if it's a symlink.  */
 virtual_ftype_t
 fhandler_netdrive::exists ()
 {