Fix slow kprop dejagnu test

Fix slow kprop dejagnu test

Fix kpropd -S -t to actually exit after processing one connection (it
was breaking out of the switch statement, not the while loop).  Use
the -t when invoking kpropd from the dejagnu test framework;
previously it was unnecessary because kpropd -S -d exited after one
connection.  Clear up some confusion in the kprop.exp comments about
whether kpropd is expected to exit.