negcache: Soften condition for expired entries
    
    Type of timestamp for entries in negative cache is time_t
    which is number of *seconds* that have elapsed since 1 January 1970.
    
    The condition for ttl was to strict so entry could be valid
     from  "ttl-1" to  ttl e.g.
     * ttl is 1 second
     * entry was stored to negative cache at 1432120871.999639
       stored_timestamp = 1432120871
     * entry was tested few miliseconds later 1432120872.001293
       current_time = 1432120872
    
    Entry was marked as expired becuase result of condition was false
      stored_timestamp + ttl < current_time
              1432120871 + 1 < 1432120872
    
    This is a reason why ./test-negcache sometime fails.
    It's quite easily reproducible on slow machine or when valgrind was used.
    
    sh$ while libtool --mode=execute valgrind ./test-negcache ; do echo OK: done
    
    Reviewed-by: Pavel Reichl <preichl@redhat.com>
    (cherry picked from commit 75e4a7753c44e9f2a7a65fad77d95e394f81c125)