Handle symlinks on windows

[Stebalien]

Now that @whyrusleeping added symlink support to tar-utils, fetching something with a symlink in it fails instead of succeeding (when it shouldn't). However, ideally, we'd do something better than simply fail...

[djdv]

Without proper access rights a user can't create a symlink. We can see if we have that right and make decisions around that. Either fail with a message alerting the user to enable symlink creation for that user, or use a secondary method such as resolving the link's target and copying the contents (with or without some depth limit to deal with recursive links).

If we decide to alert the user, we can write a section in the Windows build document that helps them to enable symlink creation for their account and then link it to them.

[djdv]

@Stebalien
Please take a look over here whyrusleeping/tar-utils#2 (comment)

[Stebalien]

Is there anything left here?

[djdv]

We have the ability to add and traverse links while adding, but we don't have a strategy merged for extracting links.
The major problem is that on Windows, we need to know what the symlinks target type is, at creation time. But we have no way to know that.
The other issue is extracting files from IPFS that have platform illegal names.
We have to decide on the expected behavior for platform specific problems such as these, and/or a way to configure that.

Context: #4956 (comment)

[Stebalien]

Got it.