sd-bus: add new call sd_bus_message_sensitive() and SD_BUS_VTABLE_SENSITIVE

This allows marking messages that contain "sensitive" data with a flag.
If it's set then the messages are erased from memory when the message is
freed.

Similar, a flag may be set on vtable entries: incoming/outgoing message
matching the entry will then automatically be flagged this way.

This is supposed to be an easy method to mark messages containing
potentially sensitive data (such as passwords) for proper destruction.

(Note that this of course is only is as safe as the broker in between is
doing something similar. But let's at least not be the ones at fault
here.)