sd-bus: follow kdbus changes (ABI break)

Implement a recent change in the kdbus pool logic:

PAYLOAD_VEC_OFF items are now referencing offsets relative to the
connection's pool, not to the item itself. Follow this change in
sd-bus.
man: fix reference to obsolete command "systemctl dump"

https://bugs.freedesktop.org/show_bug.cgi?id=87020
update TODO
net_setup/networkd: warn if matching is done on possibly unstable ifname
remove duplicated includes