Return proper exit code for TERM signal

Attempt at returning the proper exit code (128+15) when TERM signal
is sent to the server, for both single and clustered mode.

The changes are achieved by restoring signal from within the trap
and accordingly killing the process using TERM event.

Note: For clustered mode, now that we restore and return the proper
exit code, it means, every worker would also return 143 as exit code
for INT (ctrl + c) signal, earlier it was 0. Which I believe is ok, because
technically a TERM is being sent to the workers, [here](https://github.com/puma/puma/blob/master/lib/puma/cluster.rb#L56).

Happy to update/revert on any concerns :).

lib/puma/cluster.rb

@@ -262,6 +262,9 @@ def worker(index, master)
 
       Signal.trap "SIGTERM" do
         server.stop
+
+        Signal.trap("SIGTERM", "DEFAULT")
+        Process.kill("TERM", Process.pid)
       end
 
       begin

lib/puma/launcher.rb

@@ -392,7 +392,10 @@ def setup_signals
 
       begin
         Signal.trap "SIGTERM" do
-          stop
+          graceful_stop
+
+          Signal.trap("SIGTERM", "DEFAULT")
+          Process.kill("TERM", Process.pid)
         end
       rescue Exception
         log "*** SIGTERM not implemented, signal based gracefully stopping unavailable!"