Cygwin: fhandler_base::fstat_fs: accomodate the O_PATH flag

Cygwin: fhandler_base::fstat_fs: accomodate the O_PATH flag

Treat a special file opened with O_PATH the same as a regular file,
i.e., use its handle to get the stat information.

Before this change, fstat_fs opened the file a second time, with the
wrong flags and without closing the existing handle.  A side effect
was to change the openflags of the file, possibly causing further
system calls to fail.

Currently this change only affects FIFOs, but it will affect
AF_LOCAL/AF_UNIX sockets too once they support O_PATH.