ipa-kdb: handle dates up to 2106-02-07 06:28:16
    
    krb5 uses the negative part of krb5_timestamp to store time values
    after 2038:
    https://k5wiki.kerberos.org/wiki/Projects/Timestamps_after_2038
    In other words, krb5 uses krb5_timestamp (signed int) with
    unsigned arithmetic for expanding the timestamp's upper bound.
    
    This commit:
      - adds some helper functions for working with krb5_timestamp as
        unsigned (actually copied from
        https://github.com/krb5/krb5/blob/master/src/include/k5-int.h)
      - replaces operations with krb5_timestamp's by these new functions
    
    Fixes: https://pagure.io/freeipa/issue/8028
    Signed-off-by: Slava Aseev <ptrnine@altlinux.org>
    Reviewed-By: Alexander Bokovoy <abokovoy@redhat.com>
    Reviewed-By: Robbie Harwood <rharwood@redhat.com>