libafs: allow bkg daemon requests without creds

Make the creds argument optional for background daemon
requests which do not need to pass a cred.

Change-Id: Ic4ac69d746e8a84993069e37bdd0440622febd70
Reviewed-on: http://gerrit.openafs.org/10880
Tested-by: BuildBot <buildbot@rampaginggeek.com>
Reviewed-by: D Brashear <shadow@your-file-system.com>