jobcontrol: Fork bomb prevental

jobcontrol: Fork bomb prevental

If something happens that prevents existing jobs from terminating, it
is generally bad practice to keep on forking more processes
indefinitely. To alleviate the problem, implement a somewhat trivial
process limitation feature. This will define a limit of maximum number
for the processes pending execution. If the number exceeds the global
limit, new processes are to be terminate itself immediately.

This however prevents only "limited forks" for creating too many
processes for waiting execution. Normal forks still have no
limit. This should still make the system to stay alive longer during
situations where something prevents (temporarily) executing processes
to finish in time.

Signed-off-by: Timo Kokkonen <timo.t.kokkonen@iki.fi>