WasmFS: How much POSIX magic directory stuff do we want?

E.g. `test_proc_self_fd` tests for opening the directory `/proc/self/fd/`, under which each file descriptor appears as a file. I suspect this is needed pretty rarely, and should probably be an opt-in flag for maximal POSIX support or such.

Perhaps the `/tmp/` and `/dev/[null|stdout|random..]` files/directories could also be behind such a flag, but those do seem like they are much more commonly used, and we support them atm.