[job] Allow job_progress() to return an ongoing job status code, if known

[job] Allow job_progress() to return an ongoing job status code, if known

Some background jobs have a meaningful ongoing status code (e.g. the
current link status for a job waiting for a network link to come up).
Allow this to be exposed via the job_progress() method.

Signed-off-by: Michael Brown <mcb30@ipxe.org>