Add a new kernel option, MALLOC_MAKE_FAILURES, which compiles

in a debugging feature causing M_NOWAIT allocations to fail at
a specified rate.  This can be useful for detecting poor
handling of M_NOWAIT: the most frequent problems I've bumped
into are unconditional deference of the pointer even though
it's NULL, and hangs as a result of a lost event where memory
for the event couldn't be allocated.  Two sysctls are added:

debug.malloc.failure_rate

  How often to generate a failure: if set to 0 (default), this
  feature is disabled.  Otherwise, the frequency of failures --
  I've been using 10 (one in ten mallocs fails), but other
  popular settings might be much lower or much higher.

debug.malloc.failure_count

  Number of times a coerced malloc failure has occurred as a
  result of this feature.  Useful for tracking what might have
  happened and whether failures are being generated.

Useful possible additions: tying failure rate to malloc type,
printfs indicating the thread that experienced the coerced
failure.

Reviewed by:	jeffr, jhb

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