unknown flow_sample_element: 0:2048

When using NTOP's sflow plugin, samples of ethernet packets aren't properly processed.
All packets are registered as non-IP, and a message "unknown flow_sample_element: 0:2048" is generated for every sample.

I found that this is happening because the SFlow Datagram have the MAC Address registered as a 8 bytes, while code treats it as only 6 bytes.

Also, when debugging sflow plugin, mac addresses bytes are written as signed integers, instead of unsigned char.

I suggest that sflow code (plugins/sflowplugin.c) to be changed this way:

---------------------
< if(SFLOW_DEBUG(deviceId)) traceEvent(CONST_TRACE_INFO, "dstMAC %02x%02x%02x%02x%02x%02x\n", ptr[0], ptr[1], ptr[2], ptr[3], ptr[4], ptr[5]);
---
> if(SFLOW_DEBUG(deviceId)) traceEvent(CONST_TRACE_INFO, "dstMAC %02hhx%02hhx%02hhx%02hhx%02hhx%02hhx\n", ptr[0], ptr[1], ptr[2], ptr[3], ptr[4], ptr[5]);
1613c1613
< if(SFLOW_DEBUG(deviceId)) traceEvent(CONST_TRACE_INFO, "srcMAC %02x%02x%02x%02x%02x%02x\n", ptr[0], ptr[1], ptr[2], ptr[3], ptr[4], ptr[5]);
---
> if(SFLOW_DEBUG(deviceId)) traceEvent(CONST_TRACE_INFO, "srcMAC %02hhx%02hhx%02hhx%02hhx%02hhx%02hhx\n", ptr[0], ptr[1], ptr[2], ptr[3], ptr[4], ptr[5]);
1881c1881
< sprintf(buf, "%02x%02x:%02x%02x:%02x%02x:%02x%02x:%02x%02x:%02x%02x:%02x%02x:%02x%02x",
---
> sprintf(buf, "%02hx%02hx:%02hx%02hx:%02hx%02hx:%02hx%02hx:%02hx%02hx:%02hx%02hx:%02hx%02hx:%02hx%02hx",
2371c2371
< skipBytes(sample, 6);
---
> skipBytes(sample, 8);
2373c2373
< skipBytes(sample, 6);
---
> skipBytes(sample, 8);
2378c2378
< if(SFLOW_DEBUG(deviceId)) traceEvent(CONST_TRACE_INFO, "ethernet_src %02x%02x%02x%02x%02x%02x\n", p[0], p[1], p[2], p[3], p[4], p[5]);
---
> if(SFLOW_DEBUG(deviceId)) traceEvent(CONST_TRACE_INFO, "ethernet_src %02hhx%02hhx%02hhx%02hhx%02hhx%02hhx\n", p[0], p[1], p[2], p[3], p[4], p[5]);
2380c2380
< if(SFLOW_DEBUG(deviceId)) traceEvent(CONST_TRACE_INFO, "ethernet_dst %02x%02x%02x%02x%02x%02x\n", p[0], p[1], p[2], p[3], p[4], p[5]);
---
> if(SFLOW_DEBUG(deviceId)) traceEvent(CONST_TRACE_INFO, "ethernet_dst %02hhx%02hhx%02hhx%02hhx%02hhx%02hhx\n", p[0], p[1], p[2], p[3], p[4], p[5]);
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