Change name 'av_top' to 'av_top_index', 'av_tindex'

In using the av_top() function created in a recent commit, I found
myself being confused, and thinking it meant the top element of the
array, whereas it really means the index of the top element of that
array.  Since the new name has not appeared in a stable release, it can
be changed, without remorse, to include 'index' in it.
Change pods to not refer to av_len()

This name for the function is misleading; don't encourage its use.