[PATCH] Improve free drive letter detection for windows tests

Hi,

the current test code causes two test failures on my machine (Windows,
python 64-bit, drives G-J HDDs, drive K dvd drive - tested with SVN
1.9.4 and trunk).
The issue seems to be that the fallback detection code incorrectly
concludes that drive K is a free drive letter, if there's no disc in the
drive. This then results in two test failures for me:

- checkout #14
- update #31

Full details available here [1].

The attached patch resolves the issue by replacing the current code
detecting free drive letters by using the ctypes library instead of the
win32api library.

As far as I understand things, this is backwards compatible with Python
>= 2.5 [2]. Since SVN 1.9/trunk require Python >= 2.7, I don't see any
compatibility concern for this change.

[[[
Resolve test failures on Windows when running with Python 64-bit.

* subversion/tests/cmdline/checkout_tests.py
  (checkout_wc_from_drive): replace the win32api-dependent drive letter
                            detection code with the an approach using
the more
                            portable ctypes library and direct call to the
                            kernel32's GetLogicalDrives() function
* subversion/tests/cmdline/update_tests.py
  (update_wc_on_windows_drive): the same
]]]

[1] http://www.luke1410.de:8090/browse/MAXSVN-65
[2] http://python.net/crew/theller/ctypes/

Regards,
Stefan