Add a new sysctl, debug.kdb.stop_cpus, which controls whether or not we
attempt to IPI other cpus when entering the debugger in order to stop
them while in the debugger. The default remains to issue the stop;
however, that can result in a hang if another cpu has interrupts disabled
and is spinning, since the IPI won't be received and the KDB will wait
indefinitely. We probably need to add a timeout, but this is a useful
stopgap in the mean time.

Reviewed by: marcel