Minimize race conditions in t_iprop.py

Augment kpropd's debugging output so we can see the starting and
ending serial numbers of a sync, and check them in t_iprop.py's
wait_for_prop to make sure we're observing the correct sync each time.
Try to make sure we only send SIGUSR1 when kpropd is actually
sleeping, by waiting for it to print its pre-sleep message to stderr
after a full dump and after each sync.  This isn't perfect, as there
is a short window between printing to stderr and actually sleeping,
but we can't do any better until kpropd has a more reliable mechanism
for triggering immediate updates.