sd-bus: allow setting a per-connection default value for the "allow-interactive-authe...

sd-bus: allow setting a per-connection default value for the "allow-interactive-authentication" message flag

Most of our client tools want to set this bit for all their method
calls, even though it defaults to off in sd-bus, and rightfully so.
Hence, to simplify thing, introduce a per sd_bus-object flag that sets
the default value for all messages created on the connection.