rtnl: internal headers must include pragma once protection
rtnl: headers in src/systemd/ may not include internal headers

Hence including "util.h" from sd-rtnl.h is not OK. Let's minimize our
headers we pull in a bit.
rtnl: headers in src/systemd/ may not use #pragma once
api: add C++ guards
bus: indicate in .pc file that we need libsystemd-id128 definitions