updating mkreadme for packages to use index.html

[nia <nia%NetBSD.org@localhost>]

recently i've been maintaining mkreadme a bit, you can see the
fruits of my labour here:

http://cdn.netbsd.org/pub/pkgsrc/current/pkgsrc/README.html

i would like to update the script so the filenames it generates
are named index.html instead of README.html, as this is mostly
what web servers and browsers expect to be serving for a directory
display, and i think the URLs are much nicer.

this would mean that e.g. the raw filenames in
http://cdn.netbsd.org/pub/pkgsrc/current/pkgsrc/news/

are no longer direcctly browsable, but i suspect anyone
looking for raw pkgsrc recipes is more interested in either
the version control repository or pkgsrc.tar.gz.