[WIP] Symlinks

There are still some things to be resolved, but does this seem like a good way to implement it?

My idea is that instead of adding system calls (symlink and readlink), symlinks can be opened with an O_SYMLINK flag to read where they point to, and created by opening with that flag and writing a path.

A limitation of the current implementation is that it only works with symlinks to the same scheme, but I don't think that should be the case. How should that be handled? It could be useful to have some way for open() to defer to another scheme.