rcuperf: Set more user-friendly defaults

Common-case use of rcuperf must set rcuperf.nreaders=0 and if not built
as a module, rcuperf.shutdown.  This commit therefore sets the default
for rcuperf.nreaders to zero and sets the default for rcuperf.shutdown
to zero if rcuperf is built as a module and to one otherwise.

Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>