Added a run-time check to warn if TFTP is going to fail due to portability

issues in the code.

docs/KNOWN_BUGS

@@ -3,6 +3,8 @@ join in and help us correct one or more of these! Also be sure to check the
 changelog of the current development status, as one or more of these problems
 may have been fixed since this was written!
 
+28. The TFTP code is not portable and will fail on some architectures.
+
 26. NTLM authentication using SSPI (on Windows) when (lib)curl is running in
   "system context" will make it use wrong(?) user name - at least when compared
   to what winhttp does. See http://curl.haxx.se/bug/view.cgi?id=1281867

lib/tftp.c

@@ -529,6 +529,25 @@ CURLcode Curl_tftp_connect(struct connectdata *conn, bool *done)
   tftp_state_data_t     *state;
   int rc;
 
+  /*
+   * The TFTP code is not portable because it sends C structs directly over
+   * the wire.  Since C gives compiler writers a wide latitude in padding and
+   * aligning structs, this fails on many architectures (e.g. ARM).
+   * 
+   * The only portable way to fix this is to copy each struct item into a
+   * flat buffer and send the flat buffer instead of the struct.  The
+   * alternative, trying to get the compiler to eliminate padding bytes
+   * within the struct, is a nightmare to maintain (each compiler does it
+   * differently), and is still not guaranteed to work because some
+   * architectures can't handle the resulting alignment.
+   *
+   * This check can be removed once the code has been fixed.
+   */
+  if(sizeof(struct tftp_packet) != 516) {
+    failf(conn->data, "tftp not supported on this architecture");
+    return CURLE_FAILED_INIT;
+  }
+
   if((state = conn->proto.tftp = calloc(sizeof(tftp_state_data_t), 1))==NULL) {
     return CURLE_OUT_OF_MEMORY;
   }