Add an "Interface.FlushBSS" method to the new D-Bus API and a "flush"
method to the old API. Both take an age parameter that is a threshold
(in seconds) for selecting entries to evict. Setting this parameter
to zero flushes all entries from the cache.
This mechanism is useful for a connection manager to clear state at
startup and on resume (where the age parameter may be used to hold
onto recent/valid data).
These header files are included outside the dbus subdirectory and there
is not really any need to force the libdbus dbus/dbus.h header file to
be included into these files.
