Run processes in new pgroup

Some processes are getting orphaned when running Foreman with JRuby.
Creating a new pgroup allows them all to be killed together.

I believe the issue is related to how JRuby handles `Dir.chdir` by
creating a shell process: `sh -c 'cd /chdir/target; ${command}'`. That
causes a second process to be created that won't get cleaned up by
killing the parent.

lib/foreman/engine.rb

@@ -191,14 +191,14 @@ def kill_children(signal="SIGTERM")
       @running.each do |pid, (process, index)|
         system "sending #{signal} to #{name_for(pid)} at pid #{pid}"
         begin
-          Process.kill(signal, pid)
+          Process.kill("-#{signal}", pid)
         rescue Errno::ESRCH, Errno::EPERM
         end
       end
     else
       begin
         pids = @running.keys.compact
-        Process.kill signal, *pids unless pids.empty?
+        Process.kill("-#{signal}", *pids) unless pids.empty?
       rescue Errno::ESRCH, Errno::EPERM
       end
     end

lib/foreman/process.rb

@@ -49,9 +49,10 @@ def run(options={})
     env    = @options[:env].merge(options[:env] || {})
     output = options[:output] || $stdout
     runner = "#{Foreman.runner}".shellescape
-
+    pgroup = Foreman.windows? ? :new_pgroup : :pgroup
+
     Dir.chdir(cwd) do
-      Process.spawn env, expanded_command(env), :out => output, :err => output
+      Process.spawn env, expanded_command(env), :out => output, :err => output, pgroup => true
     end
   end