fix: report queue post-job status correctly

[Alkapuce]

Fixes #92

Problem

A normally completed job can still show Post job process failed in bhist -l <jobid>.

Root Cause

When neither the queue nor the job configures POST_EXEC, mbd.job.c stores an empty postCmd. finishJob() still calls runQPost(), whose previous implementation returned -1 for an empty command. This caused the post-job process to exit with a non-zero status, so jobGone() set JOB_STAT_PERR and bhist reported Post job process failed.

The existing parent-process path also returned -1 after waiting, even when the post-exec command exited successfully.

Changes

• Treat an empty POST_EXEC as success.
• Retry waitpid() when interrupted by EINTR.
• Return success only when the post-exec command exits with status 0.
• Report non-zero exits, signal termination, and wait failures as errors.
• Correct the qpre wait error message to qpost.

Validation

• Clean Ubuntu 20.04 container
• ./bootstrap.sh
• ./configure --prefix=/tmp/volclava-install
• make -j2
• make check
• git diff --check
• git apply --check issue-92-fix.patch

All completed successfully.

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