rpm: include RPM macro definition for .spec snippets

Let's try to standardize a bit the RPM macros used for
installing/uninstalling services.

This only covers the non-SysV compat bits, since that tends to vary
widely between the various distros.

Usage:

Add %{?systemd_requires} to the header of the spec file. And then:

%post
%systemd_post foobar.service

%preun
%systemd_preun foobar.service

%postun
%systemd_postun foobar.service

And, instead of the latter, in case the service shall be restarted on updates:

%postun
%systemd_postun_restart foobar.service

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