rpm: don't hardcode the binary paths in the macros, rely on $PATH

this gives us a little bit more freedom to move things around later on,
as we don't hardcode the systemd paths in old RPMs that shall work with
new systemds.
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.
sysusers: always treat ENOENT as entry-not-found when doing NSS calls

For most NSS calls it is documented that they return NULL + errno=0 when
an entry is not found. However, in reality it appears to be common to
return NULL + errno=ENOENT, instead. Handle that correctly, and don't
consider ENOENT a systematic error.
os-release: define /usr/lib/os-release as fallback for /etc/os-release

The file should have been in /usr/lib/ in the first place, since it
describes the OS container in /usr (and not the configuration in /etc),
hence, let's support os-release files in /usr/lib as fallback if no
version in /etc exists, following the usual override logic.

A prior commit already enabled tmpfiles to create /etc/os-release as a
symlink to /usr/lib/os-release should it be missing, thus providing nice
compatibility with applications only checking in /etc.

While it's probably a good idea if all apps check both locations via a
fallback logic, it is only necessary in the early boot process, as long
as the /etc/os-release symlink has not been restored, in case we boot
with an empty /etc.
tmpfiles: always use relative symlinks from tmpfiles snippets
core: don't complain at early boot if /etc/mtab is not the right symlink

When we boot up with an empty /etc it's ok if the symlink doesn't exist.
We will create it later with tmpfiles.