This adds JRuby's logic used on platforms where we do not have
native access to posix_spawn and related posix functions needed
to do fully-native subprocess launching and management. The code
here instead uses the JDK ProcessBuilder logic to simulate most
of the Open3 functionality.

This code does not pass all tests, currently, but provides most of
the key functionality on pure-Java (i.e. no native FFI) platforms.

https://github.com/ruby/open3/commit/689da19c42