DTLS examples cannot send message to localhost

[pokgak]

Description

The current dtls-echo and dtlsc-sock cannot send messages to localhost (::1).

Steps to reproduce the issue

1. Start a dtls-echo/dtls-sock example application.
2. Start the server: dtlss start
3. Send a message to localhost: dtlsc ::1 HELLO

Expected results

The sent message echoed back.

Actual results

For dtls-echo:

> dtlss start
dtlss start
> dtlsc ::1 HELLO
dtlsc ::1 HELLO
Received error during message handling

For dtls-sock:

> dtlss start
dtlss start
> dtlsc ::1 HELLO
dtlsc ::1 HELLO
# long hang, manually cancelled after 2 minutes

Versions

master branch with commit 2bdf9b1