* dtls/__init__.py: import error codes from err.py as error_codes for external access
* dtls/err.py: Added errors for ERR_WRONG_SSL_VERSION, ERR_CERTIFICATE_VERIFY_FAILED, ERR_NO_SHARED_CIPHER and ERR_SSL_HANDSHAKE_FAILURE
* dtls/openssl.py:
- Added constant SSL_BUILD_CHAIN_FLAG_NONE for SSL_CTX_build_cert_chain()
- Added method SSL_get_peer_cert_chain()
* dtls/patch.py: Added getpeercertchain() as method to ssl.SSLSocket()
* dtls/sslconnection.py:
- Bugfix SSLContext.set_ecdh_curve() returns 1 for success and 0 for failure
- SSLContext.build_cert_chain() changed default flags to SSL_BUILD_CHAIN_FLAG_NONE
- In SSLConnection() the mtu size gets only set if no user config function is given
- SSLConnection.listen() raises an exception for ERR_WRONG_VERSION_NUMBER, ERR_COOKIE_MISMATCH, ERR_NO_SHARED_CIPHER and all other unknown errors
- SSLConnection.read() and write() now can also raise ERR_PORT_UNREACHABLE
- If SSLConnection.write() successfully writes bytes to the peer, then the handshake is assumed to be okay
- Added method SSLConnection.getpeercertchain()
* dtls/test/unit.py: ThreadedEchoServer() with an extra exception branch for the newly raised exceptions in SSLConnection.listen()