Windows has a limit of 260 characters on normal paths, but it's possible
to use longer paths by using "extended-length paths" that begin with
`\\?\`. This commit attempts to transparently convert an absolute path
to an extended-length path, following the subtly different rules those
paths require. It does not attempt to handle relative paths, which
continue to be passed to the operating system unmodified.

This adds a new test, TestLongPath, to the os package. This test makes
sure that it is possible to write a path at least 400 characters long
and runs on every platform. It also tests symlinks and hardlinks, though
symlinks are not testable with our builder configuration.

HasLink is moved to internal/testenv so it can be used by multiple tests.

https://msdn.microsoft.com/en-us/library/windows/desktop/aa365247(v=vs.85).aspx
has Microsoft's documentation on extended-length paths.

Fixes #3358.
Fixes #10577.
Fixes #17500.

Change-Id: I4ff6bb2ef9c9a4468d383d98379f65cf9c448218
Reviewed-on: https://go-review.googlesource.com/32451
Run-TryBot: Quentin Smith <quentin@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Russ Cox <rsc@golang.org>