[pca] specifying a specific date instead of minage

[amy.rich at tufts.edu]

We have different classes of machines, dev, test, staging, and production,
upon which we install patches in a progressive cycle.  We use the same
pca.conf file for all systems.  I'd like to be able to freeze the date, not
minage, of patches that we install so that all levels get the same patches
regardless of the day on which they were installed.

Telling the machine not to update the patchdiag.xref file with --nocheckxref
seems suboptimal.  There's no easy way to ensure that they're all looking at
the one from the same date while still keeping the copy on the proxy server
current (in case we quickly need to apply a security patch, for example).

Is there already a way to do this that I've missed?