MFC of r211190:

• Improve the wait4data() routine so it behaves better when checking print-jobs which have last-modification times that are in the future. This shouldn't happen, of course, but it can. And when it did happen, the previous check could cause completely-spooled jobs to sit in the queue for 20 minutes per job. The new code waits until the last-modify time is not changing, instead of making decisions based on the specific value of last-modify.