Fix (and change) Foreach-Variable xlat.  Add tests.

This is a user-visible change.

  It was broken, you had to do %{Foreach-Variable-0:foo} rather than manpage.
     Probably the result of streamlining syntax.

  This changes it to %{Foreach-Variable:0} but some trickery in the parse
     might be able to preserve old behavior while depricating it.

  Since it already was an xlat and not a virtual attribute, nothing
     changes on that level.

On a related topic, Attribute[N], Attribute[#] and Attribute[*] are broken right now, and I can't even find the code for their expansion in the current repo.  If that's not something that will be landing with more of the xlat refactor, I could work on that and add some more to this branch before the pull.  Please advise.