append index.html to url generated by urlto("")

append index.html to url generated by urlto("")

This special case crops up when generating the parentlink to the toplevel
index page. urlto("") had been generating a link to "./" (or "../" etc)
for that, which is fine, if the web server redirects that to the toplevel
index.html. It's less fine if there is no web server.

I actually ran into the problem first when using gopher. (Yes, yes, don't
laugh.. see upcoming tip.) But it also crops up when browsing local wiki
files.

Of course, the index.html is stripped back off if usedirs is enabled.