Move daemon-reload from package %post scripts to file triggers

This uses new functionality added in rpm 4.13. Instead of doing
one daemon-reload per packages, we do just one or two
(When both installing and uninstalling packages, we do
two. Unfortunately this also includes the common case of upgrades.
When only installing or when only installing, we do just one.)

New file triggers.systemd can be built, but the contents have
to be copied into the rpm spec file by hand. Using %{load} does
not seem to work. It can serve as documentation.