[MERGE] Extend detection of long paths support in selftest

[Martin (gzlist)]

I notice Vincent's new buildbot is repeatedly failing to complete
selftest under cygwin:
<http://babune.ladeuil.net:26862/builders/w2003x64>
There are some errors like:
OSError: [Errno 91] File name too long: <verylongpath>

I remember having nightmares with selftest under IronPython before
spotting a ``if sys.platform = "win32":`` I'd missed.

See the attached bundle for a fix. The first commit just makes it work
with cygwin. The second commit is an example of how some of the
filesystem difference adaptation code could work.

Currently, for instance:
* HardlinkFeature depends on the presence of os.link - and doesn't
work out that you can hardlink files on NTFS drives under XP or later
(which is fine, that'd need extra code).
* CaseInsCasePresFilenameFeature and others depend on doing some
experiments and detecting what happens (which is fine, but can be got
from the drive as well).

Anyway, just pull the first revision if the basic solution is all that's wanted.

Martin
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