Windows: fix uname on recent Windows versions

Correctly identify Windows 8.1, Windows 10, and Windows Server 2008
and later from their NT versions.

On recent Windows versions, the GetVersionEx() function may report
an earlier Windows version than the running OS. To avoid user
confusion, add "[or later]" to Tor's version string on affected
versions of Windows.

Remove Windows versions that were never supported by the
GetVersionEx() function.

Stop duplicating the latest Windows version in get_uname().

Fixes bug 28096; bugfix on 0.2.2.34; reported by Keifer Bly.