my_init: runsvdir no longer redirects stderr to proctitle.

As described at http://smarden.org/runit/runsvdir.8.html passing a log option to runsvdir causes stderr to be redirected to the proctitle.

That prevents stderr from reaching docker logs, which is a big problem.

This is a much simpler alternative to #81 which added more complexity by making this configurable.
@FooBarWidget and myself agreed this should be default; I see no good reason to make it configurable.