Usage: int dds_flush(int brc_flag);
Returns: 0 if good status, > 0 number of processes still holding events.
Flushes the DD system by:
1. disables input (all input processes will fall asleep until input
is enabled)
2. empties all non-empty fifos
3. waits until all processes attached to the DD system are no longer
processing events (or timeout). Waits a maximum of DD_FLUSH_MAXWAIT
ticks, each tick is DD_FLUSH_WAIT_TICK seconds long (as defined in
$dd_inc/dd_dcom.h).
4. enables input again.
One possible use of this routine can be to remove all existing events
in the DD system and make sure that all events in the
system are new.
If the int brc_flag is nonzero, then the flush is followed by
a broadcast command DD_BRCCMD_FLUSH (as set in dd_usr_brccmd.h).