New "warn" command line option and minor bug fixes.

New command line option "warn" for changing the warning threshold.  Default
value is 80% as before.

Update maintenance logic to include both positive and negative changes.

Include job names when printing maintenance update progress.

Fixed a minor bug where a job with a null timestamp could produce uninitialized
value warnings, by setting a numeric default value of zero.

Fixed a minor bug where exit was needed instead of return, producing a "can't
return outside a subroutine" warning.