s3:registry: Prevent creation of keys containing the '/' character.

s3:registry: Prevent creation of keys containing the '/' character.

This creates a broken registry that can only be fixed with
tdbtool, since the '/' sign is used as a key separator after
normalization at a lower level.

This makes e.g. "net conf setparm abc/def comment xyz" fail with
WERR_INVALID_PARAM, which is much more desirable than a broken
registry.tdb.

Michael