io.pathnames: Revised ~ support to expand on the first tilde, and not on "~/".

io.pathnames: Revised ~ support to expand on the first tilde, and not on "~/".

This mimics the same behaviour as "ls ~" in the shell, which expands the alias
as a directory, and not as a filename. For the specific edge-case where a
filename called "~" must have a path constructed to it (e.g. "./~" or "~/~").