use ctypes.get_last_error instead of GetLastError to access Windows error code

[davidsarah]

We have some code in src/allmydata/windows/fixups.py​ and src/allmydata/util/fileutil.py​ that calls Windows API functions, and then calls GetLastError? if the API function fails. In theory, the Python interpreter could run code in-between on the same thread that changes the error code. See ​http://bugs.python.org/issue1798.

(We don't have any code in Tahoe proper that calls C runtime library functions, so this issue only affects us for GetLastError?, not errno. It is somewhat less likely to happen with GetLastError? than with errno, at least in CPython, but still possible.)