Decrement active task count when maybe_execute exceeds max

Contributor: Jay Nelson (jaynel)
Branch: cxy-limit-bug

When maybe_execute_task and maybe_execute_pid were added,
the active pid count was managed incorrectly. The count
is optimistically incremented, but was never decremented
when the number of active processes was over the limit.
The count is now decremented as part of the 'refuse'
branch of the case statement that attempts to spawn a
new process.