Limit switch-from-interrupt warnings to once per thread to avoid an endless

Limit switch-from-interrupt warnings to once per thread to avoid an endless
loop.  Generate a DDB backtrace when it occurs.

Note from Peter's report that it is possible for the idle thread to panic
if e.g. an IPI or FAST interrupt running in the idle thread's context panics.
This can result in highly unexpected operation and needs to be addressed.

Reported-by: Peter Avalos <pavalos@theshell.com>