Fix daemon.is_fifo and .is_mq under Python 3

[Jajcus]

The 'path' parameter was not properly converted from Unicode
and the functions would always fail when a path was provided.

Test code:

#!/usr/bin/python3

import os
import posix
from systemd.daemon import _is_fifo

posix.mkfifo("test.fifo")
try:
    assert not _is_fifo(0, None)
    assert not _is_fifo(0, "test.fifo")
    fd = os.open("test.fifo", os.O_RDONLY|os.O_NONBLOCK)
    assert _is_fifo(fd, None)
    assert _is_fifo(fd, "test.fifo")
finally:
    os.unlink("test.fifo")

[keszybz]

Merged as 71d8261. Thanks!

No tabs please. I also started a 'tests' branch based on your example. Will merge it to master when there's more of them. Adding a proper test suite has been on my todo list for ever... I'd be happy to take pull requests with tests.