Multi-homed NFS client does not select correct service principal

The NFSv4.0 callback client in the Linux NFS server invokes gssproxy (somehow) to acquire the credential for its callback channel.

On multi-homed systems, GSSX_ARG_ACQUIRE_CRED always selects the principal associated with "uname -n". When creating an NFS client on alternate network interfaces, GSSX_ARG_ACQUIRE_CRED needs to select the principal associated with that interface, not the one associated with "uname -n".