Allow use of larger RADIUS attributes in krad library
    
        In kr_attrset_decode(), explicitly treat the length byte as unsigned.
        Otherwise attributes longer than 125 characters will be rejected with
        EBADMSG.
    
        Add a 253-character-long NAS-Identifier attribute to the tests to make
        sure that attributes with the maximal number of characters are working
        as expected.
    
        [ghudson@mit.edu: used uint8_t cast per current practices; edited
        commit message]
    
        ticket: 9036 (new)
    
    From upstream, needed in preparation for OAuth2 support for FreeIPA and
    SSSD.
    
    Signed-off-by: Alexander Bokovoy <abokovoy@redhat.com>