Try to allow children to shut down gracefully

Since signals will no longer be handled once foreman goes into
`terminate_gracefully`, default signal handlers are restored so as
not to cause it to get stuck in an unTERMable state.

This necessitates not using the process group for signalling
except as a last resort, as foreman itself will receive the signals
it sends. This splits `killall` into two methods, one which
signals only processes foreman itself has started, and one which
signals all processes in the process group to try to clean up
more aggressively, and then reworks `terminate_gracefully` to use
them.

lib/foreman/engine.rb

@@ -177,11 +177,11 @@ class Foreman::Engine
     end
   end
 
-  # Send a signal to all processesstarted by this +Engine+
+  # Send a signal to all processes started by this +Engine+
   #
   # @param [String] signal  The signal to send to each process
   #
-  def killall(signal="SIGTERM")
+  def kill_children(signal="SIGTERM")
     if Foreman.windows?
       @running.each do |pid, (process, index)|
         system "sending #{signal} to #{name_for(pid)} at pid #{pid}"
@@ -190,6 +190,21 @@ class Foreman::Engine
         rescue Errno::ESRCH, Errno::EPERM
         end
       end
+    else
+      begin
+        Process.kill signal, *@running.keys unless @running.empty?
+      rescue Errno::ESRCH, Errno::EPERM
+      end
+    end
+  end
+
+  # Send a signal to the whole process group.
+  #
+  # @param [String] signal  The signal to send
+  #
+  def killall(signal="SIGTERM")
+    if Foreman.windows?
+      kill_children(signal)
     else
       begin
         Process.kill "-#{signal}", Process.getpgrp
@@ -402,10 +417,10 @@ private
     @terminating = true
     if Foreman.windows?
       system  "sending SIGKILL to all processes"
-      killall "SIGKILL"
+      kill_children "SIGKILL"
     else
       system  "sending SIGTERM to all processes"
-      killall "SIGTERM"
+      kill_children "SIGTERM"
     end
     Timeout.timeout(options[:timeout]) do
       watch_for_termination while @running.length > 0