Fix unix_closedir_job incorrectly checking return value of job

[MisterDA]

LWT_UNIX_CHECK_JOB was incorrectly checking the directory stream
instead of the return value of closedir.

[raphael-proust]

Thanks for this fix! I've grepped for the LWT_UNIX_CHECK_JOB and there are a couple of suspicious

[raphael-proust]

There are a couple uses of LWT_UNIX_CHECK_JOB which could be simplified (they use job->result != 0 which should be the same as just job->result IIUC) and some that are somewhat arbitrarily specific (e.g., the check for closedir has job->result < 0 but the manpage for closedir mentions 0 on success and -1 on error, so we could either leave job->result).

Anyway, I didn't find errors in any of these other instances. So I'm merging this. (After rebase and CI.)