Gateway/UnixFS: specify/unify symlink handling

[lidel]

This is placeholder, we need UnixFS specs #331 to land first, then we will be able to refer to that in Gateway/Pathing specs.

Ref.

• https://github.com/ipfs/kubo/issues?q=is%3Aissue+is%3Aopen+symlink
• • suggestion from resolve relative symlinks server side on gateway #284