jobd: check for finished jobs in main loop

jobd: check for finished jobs in main loop

The current approach using a signal handler seems slightly more prone to
weird things happening, such as the signal handler triggering just once
for several finished child processes or whatever. I dunno. Instead, just
check for terminated processes from within the main loop. Make the main
loop delay less so we don't get slowed down too much.

Also make spawned child processes wait for just a little moment before
running them so ultra-fast quitters don't race us.

Signed-off-by: Jan Krüger <jk@jk.gs>