core: add new AcceptFD= setting to .busname units

core: add new AcceptFD= setting to .busname units

AcceptFD= defaults to true, thus making sure that by default fd passing
is enabled for all activatable names. Since for normal bus connections
fd passing is enabled too by default this makes sure fd passing works
correctly regardless whether a service is already activated or not.

Making this configurable on both busname units and in bus connections is
messy, but unavoidable since busnames are established and may queue
messages before the connection feature negotiation is done by the
service eventually activated. Conversely, feature negotiation on bus
connections takes place before the connection acquires its names.

Of course, this means developers really should make sure to keep the
settings in .busname units in sync with what they later intend to
negotiate.