Cygwin: fsync: Return EINVAL for special files.

- Unlike linux, fsync() calls FlushFileBuffers() even for special
  files. This causes the problem reported in:
    https://cygwin.com/pipermail/cygwin/2022-March/251022.html
  This patch fixes the issue.

winsup/cygwin/fhandler.cc

@@ -1750,7 +1750,7 @@ fhandler_base::utimens (const struct timespec *tvp)
 int
 fhandler_base::fsync ()
 {
-  if (!get_handle () || nohandle ())
+  if (!get_handle () || nohandle () || pc.isspecial ())
     {
       set_errno (EINVAL);
       return -1;