RFC: printers.conf integrity patch

[Matt L]

I've created a patch for scheduler which improves the operational integrity of printers.conf (and added a function to file.c in the process).  I have this patch deployed on my systems and it has been very successful.  I've included it as an attachment to the bug I filed here: http://cups.org/strfiles/3715/safe-printer-update.diff

As I'm new to cups development and patching, I would very much appreciate feedback on my patch from any experienced developers.

Thanks,
Matt