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

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

Even though "net conf setparm abc/def comment xyz" does not
create a broken registry we do not want such keys to be created.
Since we get problems accessing these with "net registry" since
the registry code treats the '/' sign as a separator as 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