rpm macros: add %sysusers_create_package

rpm macros: add %sysusers_create_package

This is close to %sysusers_create_inline and %sysusers_create that we had
already, but expects a file name and uses --replace= to implement proper
priority.

This is used like:
  %sysusers_create_package %{name} %SOURCE1
where %SOURCE1 is a file with called %{name}.conf that will be installed
into /usr/lib/sysusers.d/.

The tough part is that the file needs to be available before %prep,
i.e. outside of the source tarball. This is because the spec file is
parsed (and any macros expanded), before the sources are unpackaged.

v2:
- disallow the case case when --config-name= is given but there are no
  positional args. Most likely this would be a user error, so at least for now
  forbid it.
v3:
- replace --config-name= with --target=
- drop quotes around %1 and %2 — if necessary, the caller should add
  those.
v4:
- replace --target with --replace
- add a big comment