added append() and removed position-truncation

As per ACTION-819 I added append to DTB.  It wasn't recorded there, but
while I was doing it, I remember that I was supposed to remove the rule
that too-high indexes were changed to be only 1 too high (which was
needed for insert-before to act like append.)  Given that, it seemed
right to also remove the rule that had made indexes before the start of
a list turn into zero.  Now positions which are not actually positions
of some item in the list (ie are <0 or >= len) result in "unspecified"
semantics.  (In my Python implementation [1], they raise IndexError).

Append:
   http://www.w3.org/2005/rules/wiki/DTB#func:append

Changes:
   http://www.w3.org/2005/rules/wiki/index.php?title=DTB&diff=10077&oldid=10066

      -- Sandro

[1] http://dev.w3.org/cvsweb/2009/rif/list_builtins.py?rev=1.6&content-type=text/x-cvsweb-markup