kernel - Increase worst-case maximum exec rate

kernel - Increase worst-case maximum exec rate

* The pid reuse algorithm limits the maximum fork rate.  This limit
  was set too low.  Increase the limit from 10000/sec to 100000/sec.
  Currently our opteron maxes out at 43000/sec.

  Note that with 999999 pids and a 10-second mandatory reuse time
  floor there isn't much of a point increasing the limit beyond
  100000/sec.

  100,000/sec.  Currently our opteron maxes out at around
  43,000/sec (vfork/exec/wait3/exit of a small static binary).

* The domain reuse array was increased to 1MB to accomodate this
  change.  In addition, update the array in a cache-friendly manner.

* Modify test/sysperf/exec1 to take a nprocesses argument for the
  timing run.