core: unify the way we denote serialization attributes
libsystemd-bus: add lightweight object vtable implementation for exposing objects on the bus

This adds a lightweight scheme how to define interfaces in static fixed
arrays which then can be easily registered on a bus connection. This
makes it much easier to write bus services.

This automatically handles implementation of the Properties,
ObjectManager, and Introspection bus interfaces.
bus: fix synthetic error messages
bus: don't require that if a child object node exists its parent node must too
bus: fix GetAll() userdata passing
build-sys: add sd-bus-vtable.h header
build-sys: add a makefile target to run all tests through valgrind
journald: fix minor memory leak
tests: fix some memory leaks in tests
bus: fix missing initialization
udev: reset Linux Security Module labels if no custom rules are given