Convert by value passing of nfc_target to pointer for str_nfc_target and nfc_initiator_target_is_present

This becomes more consistent with all other pass by pointer of most structures.
Additionally, this should lessen stack memory usage, as building strings with str_nfc_target would push the target (283 bytes) plus then a copy of the info objects (up to 275) onto the stack as it dives into the sprintf functions.

Lastly, this makes my attempt at a .NET wrapper easier, as I can make passing by pointer work, but passing by value seems to bomb on the interop right now.

examples/nfc-poll.c

@@ -137,7 +137,7 @@ main(int argc, const char *argv[])
   }
 
   if (res > 0) {
-    print_nfc_target(nt, verbose);
+    print_nfc_target(&nt, verbose);
   } else {
     printf("No target found.\n");
   }