[v0.3] specify whether hard links work on directories?

[wingo]

POSIX allows for hard links between directories, and also allows the implementation to forbid them: https://pubs.opengroup.org/onlinepubs/009696699/functions/link.html

Linux forbids them, AFAIU.

Should WASI say anything more? For info, Wasmtime currently fails link-at with EPERM on my Linux machine https://man7.org/linux/man-pages/man2/link.2.html#ERRORS, when attempting to make a hard link to a directory.

[pchickey]

I can't imagine that many implementations rely on hard linking directories if its not supported on Linux, so lets take the easy way here and forbid them. We can specify the EPERM equivalent as the failure code if thats what Linux does.