Drop Windows 2000 compatibility

Windows 2000 extended support by Microsoft ended on July 13, 2010
(almost 6 years ago) [1]

One problem that MSDN doesn't mention Windows 2000 in API
compatibility. Even for functions that was available in Windows NT
4.0, for example IsDebuggerPresent: it's defined with #if _WIN32_WINNT
> 0x0400, while MSDN states that it's only available since Windows XP
[2]. It's very confusing and we should use LoadLibrary() for all
Windows API functions even CreateFile() to be Windows 2000 compatible
if we trust MSDN.

I also hardly believe that someone even test Subversion on Windows
2000 these days.

Serf doesn't support Windows 2000 (due direct usage of SSPI).

Given all above I propose to officially drop Windows 2000 support in
Subversion and cleanup some Windows 2000 compatibility code.

[1] http://blogs.technet.com/b/education/archive/2009/11/10/windows-2000-end-of-life.aspx
[2] https://msdn.microsoft.com/en-us/library/windows/desktop/ms680345(v=vs.85).aspx

-- 
Ivan Zhakov