[R6RS] Does for-each tail-call on last element?

[dyb at cs.indiana.edu]

> > It seems like an unnecessary constraint to me, but I don't care much one
> > way or the other.  If we do make this change, we can no longer claim
> > for-each returns an (the) unspecified value.
>
> Not only that, but a call to for-each might return
> 0 values or multiple values.  I'm fine with that.

Actually, I wasn't quite right.  If the input list is empty, for-each must
still return the unspecified value.

Kent