Support unprivileged symlink creation in Windows

Symlink creation on Windows has in the past basically required admin;
it’s being opened up a bit in the Creators Update, so that at least
people who have put their computers into Developer Mode will be able to
create symlinks without special privileges. (Microsoft are being
cautious about it all; the Developer Mode requirement makes it so that
it this won’t be as helpful as I’d like, but it’s still an improvement
over requiring admin.)

Because of compatibility concerns, they’ve hidden this new functionality
behind a new flag in the CreateSymbolicLink dwFlags:
SYMBOLIC_LINK_FLAG_ALLOW_UNPRIVILEGED_CREATE. So we add this flag in
order to join the party.

Older Windows doesn’t like this new flag, though, so if we encounter
ERROR_INVALID_PARAMETER we try again without the new flag.

Sources:

- https://blogs.windows.com/buildingapps/2016/12/02/symlinks-windows-10/
  is the official announcement (search for CreateSymbolicLink)

- https://news.ycombinator.com/item?id=13096354 on why the new flag.

- https://twitter.com/richturn_ms/status/818167548269051905 confirming
  that Developer Mode will still be required.

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