Fix fping interval detection and confusing documentation of ICMP checks interval

  5198:20200529:191836.432 item "xxxxxxxx:icmppingloss[,100,20,50,500]" became not supported: fping failed: /usr/sbin/fping: You need i >= 10, p >= 20, r < 20, and t >= 50
  5198:20200529:191836.432 item "xxxxxxxx:icmppingloss[,100,20,50,500]" became not supported: fping failed: /usr/sbin/fping: You need i >= 10, p >= 20, r < 20, and t >= 50

 * Comments: starting with fping (4.x), the packets interval can be 0ms, 1ms, *
 *           otherwise minimum value is 10ms                                  *
 ******************************************************************************
...
zbx_snprintf(tmp, sizeof(tmp), "%s -c1 -t50 -i0 %s", fping, dst);
...
zbx_snprintf(tmp, sizeof(tmp), "%s -c1 -t50 -i1 %s", fping, dst);
...
        else
             {
                *value = 10;