Do not default to host/ for client keytabs

Do not default to host/ for client keytabs

When the normal (acceptor) keytab is being used to obtain initial
credentials, it is reasonable to use the default hostbased service
principal (host/fully.qualified.localhost.domain) when no client
principal is given.  This behavior is not very reasonable when
the default client keytab is being used, as host/ credentials are
not normally client credentials.

Make kinit -i match up with the GSS-API behavior when client keytabs
are in use, using the name of the first entry in the keytab when
no name is explicitly given.

ticket: 7892