WIFI Shield support has some bad uses of strncpy()

From code inspection:.. These are incorrect calls to strncpy - using the length of the source string rather than the destination buffer...

```
libraries/WiFi/extras/wifiHD/src/ard_spi.c - instances of:
strncpy(keyIdx, (const char*)¶ms->param, params->paramLen);

hardware/arduino/avr/firmwares/wifishield/wifiHD/src/SOFTWARE_FRAMEWORK/COMPONENTS/WIFI/HD/wl_api.h - instances of:
strncpy(net.ssid.ssid, "My AP", strlen("My AP"));
```

Start with the listed strncpy instances in libraries/WiFi/extras/wifiHD/src/ard_spi.c and hardware/arduino/avr/firmwares/wifishield/wifiHD/src/SOFTWARE_FRAMEWORK/COMPONENTS/WIFI/HD/wl_api.h. Inspect the destination buffers and the surrounding WiFi Shield code; done means the identified calls use lengths appropriate for their destinations without introducing truncation or overflow.

Written by the indexing model from the issue text.