+ Adds a preinstall script to wipe out whatever the last .pkg
installed. This should avoid systems that have mad many updates
getting into strange states autoloading things that no longer
exist. Fixes#2963
+ Run add-shell with ${DSTVOLUME} prepended to the path - the
installer lets users intall onto any volume, so it's plausible
not installed onto /
+ Use `logger` instead of rando /tmp files for logging - stuff
should show up in Console.
+ make_pkg makes the pkg and also fish.app - the former was being
built with -j12 already, make the latter do so as well.
