Reject certain fieldnames.
Reject certain fieldnames.

A fieldname is rejected if:

  - It contains bytes between 1 and 31.

  - It contains slashes '/'.

Fieldnames end up in file names so this should fix a few issues with
filesystems.

tuulos/traildb,traildb/traildb,tuulos/traildb,traildb/traildb,tuulos/traildb,traildb/traildb,tuulos/traildb