sd-bus: move common errors src/shared/bus-errors.h → src/libsystemd/sd-bus/bus-common-errors.h

Stuff in src/shared/ should not use stuff from src/libsystemd/ really.
networkd: rename section [BridgePort] → [Bridge]

Let's stick to generic sections that describe the general technology,
instead of specific per-object sections, unless we really have a reason
to do that otherwise.
sd-bus: make BUS_ERROR_MAP_ELF_USE() use a const variable
update TODO