kernel - Improve pid-reuse algorithm, fix bug

kernel - Improve pid-reuse algorithm, fix bug

* Fix a bug where under extreme loads it was possible for a PID to be
  allocated twice.

* Implement a minimum pid-reuse delay of 10 seconds.  No pid, session id,
  or pgid will be reused for at least 10 seconds after being reaped.

  This shouldn't really be necessary but it should help scripts, particularly
  bulk builds, which rely on testing out-of-band PIDs with pwait.

* Increase PID_MAX from 99999 to 999999

Reported-by: marino