Add '-g' option to kinit, for creating an empty ccache on the client.

When '-g' is passed in, kinit creates a new credential cache for the
specified user but does not populate it with their session key and
does not request a TGT. The user's principal and the location of the
credential cache are determined as they would have been for a standard
invocation of kinit (i.e. they are derived from several sources
including krb5.conf and the command-line parameters to kinit).