runit: Add Darwin Compatibility

Not much modification is required to build and run runit on MacOS, all
that was needed was to follow runit's instructions [0] with some
guidance from homebrew's formula. [1]

0: http://smarden.org/runit/install.html
1: https://github.com/Homebrew/homebrew-core/blob/1cf1e61/Formula/runit.rb