krb5: fix entry order in MEMORY keytab
    
    Since krb5_kt_add_entry() adds new entries at the beginning of a MEMORY
    type keytab and not at the end a simple copy into a MEMORY type keytab
    will revert the order of the keytab entries. Since e.g. the sssd_krb5
    man page give hints about where to add entries into keytab files to help
    SSSD to find a right entry we have to keep the order when coping a
    keytab into a MEMORY type keytab. This patch fixes this by doing a
    second copy to retain the original order.
    
    Resolves https://fedorahosted.org/sssd/ticket/2557
    
    Reviewed-by: Lukáš Slebodník <lslebodn@redhat.com>
    Reviewed-by: Jakub Hrozek <jhrozek@redhat.com>