less structs and defines publicly exposed

- nfc_device is now an opaque type;
 - PN53x specific errors are not public anymore;
 - nfc_device_name() renamed to nfc_device_get_name() for the sake of consistency;
 - examples/*, utils/* uses the new nfc_device_get_name() function instead of access directly to struct's content;
 - new error defined: NFC_ERFTRANS for notifying about RF transmission error, its used by mifare.c to detect permissions error on mifare;
 - drivers initiator_transceive_bytes() function now returns libnfc's error code on failure (<0), and received bytes count on success (>=0);
 - remove some unused errors.

utils/nfc-mfultralight.c

@@ -219,7 +219,7 @@ main (int argc, const char *argv[])
     exit (EXIT_FAILURE);
   }
 
-  printf ("Connected to NFC device: %s\n", pnd->acName);
+  printf ("Connected to NFC device: %s\n", nfc_device_get_name (pnd));
 
   // Try to find a MIFARE Ultralight tag
   if (!nfc_initiator_select_passive_target (pnd, nmMifare, NULL, 0, &nt)) {