libsystemd-bus: factor out DBus bits out of sd_bus_get_owner()

Just a preparation for upcoming kdbus support.
libsystemd-bus: add sd_bus_translate_attach_flags()

Factor out code from sd_bus_negotiate_attach_creds() to a generic
translate function, so it can be used from other places.
libsystemd-bus: add kdbus support for sd_bus_get_owner()
libsystemd-bus: rename sd_bus_kernel_translate_request_name_flags

... to keep the namespace clean.
libsystemd-bus: sd_bus_request_name: use kdbus_translate_request_name_flags()
bus: add the ability for backends to queue to input messages at the same time

We need this so that one incoming kernel message can result in two
high-level bus messages, for the case where we synthesize NameAcquired
and NameOwnerChanged in the same instance.
bus: fix callback index when dispatching kernel messages
bus: synthesize NameLost/NameAcquired from kernel messages on top if NameOwnerChange