Add backend based on Process.spawn

[eregon]

From https://github.com/enkessler/childprocess/issues/172

[coveralls]

Coverage increased (+2.4%) to 92.727% when pulling 0eff297e3af25449c847716f7b0ac0c7aaebbc49 on eregon:process-spawn into 44227922488765ebad0c0bed0fbec586ef9f5c26 on enkessler:master.

[eregon]

TravisCI passes, except on JRuby: https://travis-ci.org/github/enkessler/childprocess/builds/759130946

[eregon]

TravisCI now passes with the latest JRuby, as good as master + actually it also works on ruby-head: https://travis-ci.org/github/enkessler/childprocess/builds/759183880 vs https://travis-ci.org/github/enkessler/childprocess/builds/759135035

[eregon]

This is almost ready now. CI: https://github.com/eregon/childprocess/runs/2869763551?check_suite_focus=true Linux & macOS pass.

Windows has only 2 failures:

Failures:

  1) ChildProcess lets a detached child live on
     Failure/Error: expect(alive?(p_pid)).to_not be true

       expected not #<TrueClass:20> => true
                got #<TrueClass:20> => true

       Compared using equal?, which compares object identity.
     # ./spec/childprocess_spec.rb:213:in `block (2 levels) in <top (required)>'

  2) ChildProcess kills the full process tree
     Failure/Error: raise msg

     RuntimeError:
       timed out after 3 seconds:

       expected: false
            got: true

       (compared using eql?)

       Diff:
       @@ -1 +1 @@
       -false
       +true
     # ./spec/spec_helper.rb:197:in `wait_until'
     # ./spec/childprocess_spec.rb:291:in `block (3 levels) in <top (required)>'
     # ./spec/childprocess_spec.rb:281:in `block (2 levels) in <top (required)>'

Finished in 22.11 seconds (files took 0.83769 seconds to load)
66 examples, 2 failures

The first one might be some issue with Process.detach on Windows. The second is the issue that there is no builtin way to kill a process group on Windows.

JRuby on Linux & macOS has only 1 failure:

   1) ChildProcess can unset env vars
     Failure/Error: expect(child_env).to_not have_key('CHILDPROCESS_UNSET')
       expected `{"ACCEPT_EULA"=>"Y", "AGENT_TOOLSDIRECTORY"=>"/opt/hostedtoolcache", "ANDROID_HOME"=>"/usr/local/lib/...", "XDG_CONFIG_HOME"=>"/home/runner/.config", "_"=>"/home/runner/.rubies/jruby-9.2.19.0/bin/bundle"}.has_key?("CHILDPROCESS_UNSET")` to be falsey, got true
     # ./spec/childprocess_spec.rb:161:in `block in <main>'
     # ./spec/childprocess_spec.rb:150:in `block in <main>'

Which is most likely a bug of JRuby (since that works fine on CRuby).

JRuby on Windows doesn't seem to work well, due to another JRuby bug, Process.waitpid2 seems to return nil for the status, which must be a JRuby bug.