Files which are installed as-is (any .service and other unit files, .conf
files, .policy files, etc), are left as is. My assumption is that SPDX
identifiers are not yet that well known, so it's better to retain the
extended header to avoid any doubt.
I also kept any copyright lines. We can probably remove them, but it'd nice to
obtain explicit acks from all involved authors before doing that.
refcnt.h only exists for cases where objects are simultaneously handled
by different threads. Otherwise it should not be used. The only case
where this applies is sd_bus, really, and pretty much none of our APIs,
since we do not claim thread-safety for them.