Revert simplifications of chess.gaviota from #1130

The resulting code is not equivalent, that is, when the full tablebase
is available ./test.py GaviotaTestCase fails with

    ERROR: test_dm_4 (__main__.GaviotaTestCase.test_dm_4)
    ----------------------------------------------------------------------
    Traceback (most recent call last):
      File "/home/niklas/Projekte/python-chess/./test.py", line 35, in _wrapper
        return f(self)
      File "/home/niklas/Projekte/python-chess/./test.py", line 4305, in test_dm_4
        dtm = self.tablebase.probe_dtm(board)
      File "/home/niklas/Projekte/python-chess/chess/gaviota.py", line 1393, in probe_dtm
        dtm = self.egtb_get_dtm(req)
      File "/home/niklas/Projekte/python-chess/chess/gaviota.py", line 1518, in egtb_get_dtm
        dtm = self._tb_probe(req)
      File "/home/niklas/Projekte/python-chess/chess/gaviota.py", line 1621, in _tb_probe
        buffer_zipped = stream.read(z)
    ValueError: read length must be non-negative or -1

Author: niklasf