[pca] specifying a specific date instead of minage

[amy.rich at tufts.edu]

fmaney> Simple, elegant solution: keep two copies of patchdiag.xref, one in
fmaney> your central repository that is your "golden image" across all of your
fmaney> environments for a specific date, and one that is current (and updated
fmaney> whenever) and local. You just need to specify which one you want when
fmaney> you run PCA.

Yeah, this is essentially what I've wound up doing.  The "golden image"
patchdiag.xref, as you call it, is pushed out via cfengine.  The actual
patching scripts that we use specify to always use that copy and to never
update it.

Thanks for everyone's feedback and suggestions.