gssd: process target= attribute in new upcall

Add processing of the "target=" attribute in the new gssd upcall.
Information in this field is used to construct the gss service name
of the server for which gssd will create a context .

This, along with the next patch handling "service=", is needed
for callback security.

For Kerberos, the NFS client will use a service principal present
in its keytab during authentication of the SETCLIENT_ID operation.
When establishing the context for the callback, the gssd on the
NFS server will attempt to authenticate the callback against the
principal name used by the client.

Note: An NFS client machine must have a keytab for the callback
authentication to succeed.

Signed-off-by: Olga Kornievskaia <aglo@citi.umich.edu>
Signed-off-by: Kevin Coffman <kwc@citi.umich.edu>
Signed-off-by: Steve Dickson <steved@redhat.com>