nfscl: Improve NFSv4 error message for NFSERR_WRONGSEC

The usual reason for an NFSv4 server replying NFSERR_WRONGSEC
to an operation is that a Kerberos credential is required.
This patch replaces a cryptic "err=10016" with a message
suggesting that a Kerberos TGT is probably needed.

MFC after: 2 weeks