Some changes fix actual problems, e.g. missing spaces in square bracket tests,
and backticks unintentionally causing code execution when intended as formatting.
Others, such as conservative quoting probably work fine in the old version in
most situations, but it's nice to have some additional safety.
Using `{ ..; }` instead of `(..)` is just a small performance enhancement.
Many of these issues were identified by shellcheck, which might be useful in CI
as well.
+ 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.
