auditing port options and following default option value changes

[Don Lewis]

I'm pretty much ready to throw in the towel on upgrading ports in place
with portupgrade and I'm planning on switching to poudriere.  As part of
this conversion, I'd like to figure out what ports have non-default
options and which options are set to non-default values.  Then I can
figure out whether I want to set each option back to default or
propagate the non-default value to poudriere.

Also, since the option files for each port contain the values for all
the options, if I have a port with one non-default option value set, how
can I follow updates to the default values of the other options?