append! non-tail recursion

[Kevin Ryde]

I noticed append! is implemented with a non-tail recursion.  Since
it's at the C level it needs quite a long list to provoke a problem,
but I thought to redo it with a loop.

The current code uses scm_last_pair, which has the effect of detecting
circular lists in the input.  Is that an intended feature, or merely a
side effect?

Plain append doesn't check for circular, I guess probably neither or
both should do it.