-p prefix
             Set prefix as the directory in which to delete files from any in-
             stalled packages which do not explicitly set theirs.  For most
             packages, the prefix will be set automatically to the installed
             location by pkg_add(1).

I'm a bit worried.  I would like to play around with syspkg for filesystem
images and the like... This, of course, means that I want to use pkg_*
scripts, not on /, but on (for instance) /mnt.

Is there a sane way to get some assurance that no package will mistakenly try
to assert that it is relative to /?

-s