Use None as the error value instead of -1.

This is a little more conventional, and I'm going to get rid of -1 in
the regex functions as well.

native/libc.c

@@ -40,7 +40,7 @@ func_realpath(PyObject *self, PyObject *args) {

  if (status == NULL) {
    debug("error from realpath()");
    return PyLong_FromLong(-1);
    Py_RETURN_NONE;
  }

  return PyString_FromString(target);

native/libc_test.py

@@ -94,19 +94,13 @@ def testRegexFirstGroupMatch(self):
        libc.regex_first_group_match('(X.)', s, 6))

  def testRealpathFailOnNonexistentDirectory(self):
	# This behaviour is actually inconsistent with GNU readlink,
	# but matches behaviour of busybox readlink
	# (https://github.com/jgunthorpe/busybox)
	self.assertEqual(
		-1,
		libc.realpath('_tmp/nonexistent')
	)

	# Consistent with GNU
	self.assertEqual(
		-1,
		libc.realpath('_tmp/nonexistent/supernonexistent')
	)
    # This behaviour is actually inconsistent with GNU readlink,
    # but matches behaviour of busybox readlink
    # (https://github.com/jgunthorpe/busybox)
    self.assertEqual(None, libc.realpath('_tmp/nonexistent'))

    # Consistent with GNU
    self.assertEqual(None, libc.realpath('_tmp/nonexistent/supernonexistent'))


if __name__ == '__main__':

tools/readlink.py

@@ -16,9 +16,9 @@ def main(argv):
  if not arg.f:
    util.error("-f must be passed")
    return 1
  for arg in argv[i:]:
    res = libc.realpath(arg)
    if res == -1:
  for path in argv[i:]:
    res = libc.realpath(path)
    if res is None:
      return 1
    print(res)
  return 0