Use index.html files as targets (instead of zero-byte param.html

Use index.html files as targets (instead of zero-byte param.html
stamp files we were using); quit doing $(RM) *.html in clean and
instead do clean using a list of the HTML files that we know we
have actually generated. Also add pi directory to list of
directory to recurse and make (had been accidentally left out for
1.71.0 build).