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Rename the KDB_STOP_NMI kernel option to STOP_NMI and make it apply to all IPI_STOP IPIs.

Change the i386 and amd64 MD IPI code to send an NMI if STOP_NMI is enabled if an attempt is made to send an IPI_STOP IPI. If the kernel option is enabled, there is also a sysctl to change the behavior at runtime (debug.stop_cpus_with_nmi which defaults to enabled). This includes removing stop_cpus_nmi() and making ipi_nmi_selected() a private function for i386 and amd64.
Fix ipi_all(), ipi_all_but_self(), and ipi_self() on i386 and amd64 to properly handle bitmapped IPIs as well as IPI_STOP IPIs when STOP_NMI is enabled.
Fix ipi_nmi_handler() to execute the restart function on the first CPU that is restarted making use of atomic_readandclear() rather than assuming that the BSP is always included in the set of restarted CPUs. Also, the NMI handler didn't clear the function pointer meaning that subsequent stop and restarts could execute the function again.
Define a new macro HAVE_STOPPEDPCBS on i386 and amd64 to control the use of stoppedpcbs[] and always enable it for i386 and amd64 instead of being dependent on KDB_STOP_NMI. It works fine in both the NMI and non-NMI cases.

$FreeBSD: src/sys/i386/conf/NOTES,v 1.1209 2005/10/24 21:04:18 jhb Exp $