dll,fuse: allow dir buffer entry invalidation

The FUSE implementation of ReadDirectory issues readdir followed
by a slew of getattr. In the current implementation if a getattr fails
the whole readdir operation fails.

This commit adds the ability to invalidate individual entries in the
directory buffer. Entries for which getattr fails are now marked invalid
rather than fail the overall ReadDirectory operation.

See #292

src/dll/library.h

@@ -66,6 +66,7 @@ PSID FspWksidGet(WELL_KNOWN_SID_TYPE WellKnownSidType);
 
 PWSTR FspDiagIdent(VOID);
 
+#define FspFileSystemDirectoryBufferEntryInvalid ((ULONG)-1)
 VOID FspFileSystemPeekInDirectoryBuffer(PVOID *PDirBuffer,
     PUINT8 *PBuffer, PULONG *PIndex, PULONG PCount);