jobs: Handle string-based job descriptors

When looking for a job using a string descriptor, e.g.

	fg %man

the relevant loop in src/jobs.c only ever exits to the err label. With
this patch, when the end condition is reached, we check whether a job
was found, and if so, set things up to exit correctly via gotit.
Multiple matches are already caught using the test in the match block.

Signed-off-by: Stephen Kitt <steve@sk2.org>
Signed-off-by: Herbert Xu <herbert@gondor.apana.org.au>