osx: create a modern package including manpages
Instead of using bdist_mpkg, we use the modern Apple-provided tools to
build an OS X Installer package directly. This has several advantages:
* Avoids bdist_mpkg which seems to be barely maintained and is hard to
use.
* Creates a single unified .pkg instead of a .mpkg.
* The package we produce is in the modern, single-file format instead of
a directory bundle that we have to zip up for download.
In addition, this way of building the package now correctly:
* Installs the manpages, bringing the `make osx`-generated package in
line with the official Mac packages we publish on the website.
* Installs files with the correct permissions instead of encoding the
UID of the user who happened to build the package.
Thanks to Augie for updating the test expectations.