rpc: add service field to new upcall

This patch extends the new upcall with a "service" field that currently
can have 2 values: "*" or "nfs". These values specify matching rules for
principals in the keytab file. The "*" means that gssd is allowed to use
"root", "nfs", or "host" keytab entries while the other option requires
"nfs".

Restricting gssd to use the "nfs" principal is needed for when the
server performs a callback to the client.  The server in this case has
to authenticate itself as an "nfs" principal.

We also need "service" field to distiguish between two client-side cases
both currently using a uid of 0: the case of regular file access by the
root user, and the case of state-management calls (such as setclientid)
which should use a keytab for authentication.  (And the upcall should
fail if an appropriate principal can't be found.)

Signed-off: Olga Kornievskaia <aglo@citi.umich.edu>
Signed-off-by: J. Bruce Fields <bfields@citi.umich.edu>
Signed-off-by: Trond Myklebust <Trond.Myklebust@netapp.com>