vos: complain if no fields are passed

to vos "setfields". It might be a misleading if it exists
sucessfully when clearly invoked wrongly.

Change-Id: Ic92f4e17fde0a0dfc182f9713350800c72fa165e
Reviewed-on: http://gerrit.openafs.org/10271
Tested-by: BuildBot <buildbot@rampaginggeek.com>
Reviewed-by: Benjamin Kaduk <kaduk@mit.edu>
Reviewed-by: Derrick Brashear <shadow@your-file-system.com>