systemd: Set service_extra_in/socket_extra_in everywhere

systemd: Set service_extra_in/socket_extra_in everywhere

It's somewhat confusing that some of the services have a
corresponding foo.service.extra.in and foo.socket.extra.in, some
have just one of the two, and some have neither.

In order to make things more approachable, make sure that both
files exists for each service.

In most cases the extra units are currently unused, so they will
just contain a comment briefly explaining their purpose and
pointing users to meson.build, where they can find more
information. The same comment is also added to the top of
extra units that already have some contents in them for
consistency.

Signed-off-by: Andrea Bolognani <abologna@redhat.com>
Reviewed-by: Michal Privoznik <mprivozn@redhat.com>