[pca] specifying a specific date instead of minage

[amy.rich at tufts.edu]

cf2688> I keep the proxy server's copy frozen by including nocheckxref in its
cf2688> config, and update manually for regular releases or as necessary.

If you're always using an out of date patchdiag.xref file, how can you tell
what patches you're behind on on every client machine?  I think the main issue
I'm having is that I want to compare to BOTH the latest and a specified
baseline on every client.  

I want to be able to run pca -l on every client machine to generate a list of
patches that compares against the current patchdiag.xref.  This gives me a
good idea of how far each machine is behind and what they require (which then
automatically gets shoved into a database so we can easily see what machines
are lacking what).  But when it comes to installing patches, I want to be able
to specify a command line flag that says "only install patches older than this
date."  I'd also like to be able to run pca -l with a command line flag for
the date to see what patches WILL be installed with that flag.

My approach would allow:

* information about how machines compare to the latest released patches
* baseline install consistency during a patch cycle
* information about how machines compare to said baseline during a patch cycle
* the flexibility of installing a specific patch on a given machine when the
  need arises

I don't see a way to accomplish those goals by using different frozen versions
of the patchdiag.xref file.

Since the patchdiag.xref file already lists patches by release date (not age),
I was hoping this might already be part of pca's functionality (or maybe
easily added if it's not).