Code clean-up, minor IPI work, and implement more functions.

Code clean-up, minor IPI work, and implement more functions.

Remove a bunch of debug kprintfs sprinkled about.  Remove my earlier
attempts at an IPI signal handler.  Do some error checking for sigaction
and pthread_kill return values.  Always increment gd_intr_nesting_level
and remove RQF_IPIQ from the IPI signal handler.

Implement stop_cpus() and restart_cpus() using SIGSTOP and SIGCONT (this
may not be the best choice).