fix canonicalize of libc:

Previous libc: implementation was not working, I think I did not push a commit. This MR should fix the problem. Canonicalize now checks for "libc:".

If a user changes the value of $TTY, it must be a valid pathname for syscall::open, and it must not be a cross-device link. So e.g. export TTY=file:/dev/null will fail with ENOENT, but export TTY=null: will work.

I tested changing $TTY to another path on Ion. It works for non-file: schemes, but it doesn't work for file: because Ion strips the prefix.

On Bash, setting $TTY to a file: path doesn't work in the current shell, but if you set it then spawn a new shell, it works. I think the libc: scheme is not picking up bash's environment variable changes. I am not sure why.