Incremental pbulk builds

[Jason Bacon <outpaddling%yahoo.com@localhost>]

I'm trying, for the first time, to run a full bulk build following a 
major cvs update.  Until now, I've built everything
from a fresh tree.  This time I'm rebuilding -current and there have 
been some core updates since the last build.

I'm wondering if there's a canonical way to deal with the situation 
where there has been an upgrade to something
that's already in the bootstrap kit.  I can figure out how to get around 
it manually, but before I do that, is there
a simple setting in pbulk.conf or something like that?  I'm not seeing 
anything in the docs or anything that sticks
out in pbulk.conf, but maybe I'm just overlooking it.

My bulk build craps out because it can't overwrite cwrappers.  The main 
thing is to fix this in a way that won't cause pbulk
to rebuild packages that have not been upgraded, which would take a week.

Thanks.

=> Creating binary package 
/tmp/pkgbuild/pkgtools/cwrappers/work/.packages/cwrap
pers-20180325.tgz
pkg_add: A different version of cwrappers-20180325 is already installed: 
cwrappers-20171108
pkg_add: 1 package addition failed