Ensure foreman is the process group leader

Foreman should be the process leader before killing processes using
his process group id.

Before that foreman was broken when it was not spawned from a shell.

lib/foreman/engine.rb

@@ -37,6 +37,9 @@ class Foreman::Engine
   # Start the processes registered to this +Engine+
   #
   def start
+    # Make sure foreman is the process group leader.
+    Process.setpgrp unless Foreman.windows?
+
     trap("TERM") { puts "SIGTERM received"; terminate_gracefully }
     trap("INT")  { puts "SIGINT received";  terminate_gracefully }
     trap("HUP")  { puts "SIGHUP received";  terminate_gracefully } if ::Signal.list.keys.include? 'HUP'
@@ -109,7 +112,7 @@ class Foreman::Engine
       end
     else
       begin
-        Process.kill "-#{signal}", Process.pid
+        Process.kill "-#{signal}", Process.getpgrp
       rescue Errno::ESRCH, Errno::EPERM
       end
     end