Change job_signal() to job_signal_id() and _job_signal() to job_signal().

Most calling paths to job_signal() were sending in job_ptr->job_id alongside
uid 0. This bypasses the unneeded uid validation, and forces another lookup
through find_job_record() for a job_ptr we just dereferenced.

So - change the less-used job_signal() to job_signal_id(), promote
_job_signal to an extern taking that name over, and stop this whole
unnecessary extra set of lookup calls.