* upstart/message.h: Rename UPSTART_JOB_INVALID to
UPSTART_JOB_UNCHANGED, as it's not really invalid just a no-op
* upstart/message.c (upstart_message_new, upstart_message_handle):
Update the constant, fields are unchanged,
* upstart/tests/test_message.c (test_new, my_handler) 
(test_handle): Rename constants/messages.