Change av_foo_nomg() name

Change av_foo_nomg() name

These names sparked some controversy when created:

http://www.nntp.perl.org/group/perl.perl5.porters/2016/03/msg235216.html

I looked through existing code for paradigms to follow, and found some
occurrences of 'skip_foo_mg'.  So this commit changes the names to be

    av_top_index_skip_len_mg()
    av_tindex_skip_len_mg()

This is explicit about the type of magic that is ignored, and will still
be valid if another type of magic ever gets added.