Support for user-only permissions

[sunfishcode]

As discussed in #74, change-file-permissions-at is currently meant to change all the file permissions. That's enough for the use cases that motivated it. However, bytecodealliance/preview2-prototyping#37 asks if it's possible to support more fine-grained permissions, in particular changing a file to be only user-readable.

This is a little subtle because WASI has no concept of a "user", and two instances running within the same host "user" are still expected to be sandboxed from each other, and "user" permissions will be accessible to both. Also, some filesystems won't support user permissions, but some filesystems won't support any kind of permissions at all. We should document that the actual protection offered by this function entirely depends on the underlying filesystem. Nevertheless, there are use cases where this functionality may be useful.

The main thing this needs is a proposal for an API that would provide the needed functionality and a sketch of what it would do on Windows.