Remove stale Runit service symlinks

Fixes #653 (closed), I think.

If a line of Procfile gets commented out or removed, we used to not delete the corresponding Runit service. This MR changes that: we now remove the symlink from /services.

While working on this it became apparent that it's easy to forget that we use 2-element arrays as "service tuples" in lib/runit/config.rb. To make that easier to use, I changed services to being structs.