Quit with an error status on SIGTERM during migration

This prevents the supervisor from thinking that the migration completed
successfully.

In order to do this, I've introduced a new lock around the start (and
finish) of the migration so that we avoid a race between the signal
handler in the server_accept loop and the control thread mirror startup.
Without that, we'd risk successfully starting a migration after the
SIGTERM handler fired, which would be Bad.

tests/acceptance/test_dest_error_handling.rb

@@ -28,6 +28,11 @@ class TestDestErrorHandling  < Test::Unit::TestCase
   end
 
 
+  def test_sigterm_has_bad_exit_status
+    @env.nbd1.can_die(1)
+    run_fake( "source/sigterm_after_hello" )
+  end
+
   def test_disconnect_after_hello_causes_error_not_fatal
     run_fake( "source/close_after_hello" )
     assert_no_control