rpm: add RPM macros to apply sysusers, sysctl, and binfmt drop-ins

rpm: add RPM macros to apply sysusers, sysctl, and binfmt drop-ins

With this in place RPMs can make sure that whatever they drop in is
immeidately applied, and not delayed until next reboot.

This also moves systemd-sysusers back to /usr/bin, since hardcoding the
path to /usr/lib in the macros would mean compatibility breaks in
future, should we turn sysusers into a command that is actually OK for
people to call directly. And given that that is quite likely to happen
(since it is useful to prepare images with its --root= switch), let's
just prepare for it.