compute: disallow output files that are not regular files

compute: disallow output files that are not regular files

Use case where this came up is a compute program using singularity,
where the process inside the container will be allowed to write to the temp
directory, so could make eg a /etc/shadow symlink, which could then be
used to exfiltrate that from the system to wherever the annex object
might be pushed to.

It seemed better to fix this once in git-annex rather than in any such
compute program.