Implement "{default_}job_pre_exec" and "{default_}job_post_exec"

Implement "{default_}job_pre_exec" and "{default_}job_post_exec"

When the pre-exec command returns non 0, the backup job will be
skipped and listed as failed (which results in the "there have been
errors" summy); if it returns 99, the backup job is skipped but NOT
accounted as failed.

The exit code of the post-exec command is ignored.