sd-bus: introduce new sd_bus_flush_close_unref() call

sd-bus: introduce new sd_bus_flush_close_unref() call

sd_bus_flush_close_unref() is a call that simply combines sd_bus_flush()
(which writes all unwritten messages out) + sd_bus_close() (which
terminates the connection, releasing all unread messages) +
sd_bus_unref() (which frees the connection).

The combination of this call is used pretty frequently in systemd tools
right before exiting, and should also be relevant for most external
clients, and is hence useful to cover in a call of its own.

Previously the combination of the three calls was already done in the
_cleanup_bus_close_unref_ macro, but this was only available internally.

Also see #327