[SV 56918] Compute job slots properly on failing command

[SV 56918] Compute job slots properly on failing command

Ensure we properly reduce job_slots_used if a command fails because
it doesn't exist/can't be started.

* src/job.h (struct child): Add a field jobslot to be set when using
a job slot.
* src/job.c (start_waiting_job): Remember if we are using a job slot.
(reap_children): Reduce number of job slots used by jobslot.