Added more on error evaluation and a method to get the peer certificate chain

* 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()

dtls/test/unit.py

@@ -532,18 +532,20 @@ class ThreadedEchoServer(threading.Thread):
                 if acc_ret:
                     newconn, connaddr = acc_ret
                     if test_support.verbose and self.chatty:
-                        sys.stdout.write(' server:  new connection from '
-                                         + str(connaddr) + '\n')
-                    handler = self.ConnectionHandler(self, newconn)
-                    handler.start()
-            except socket.timeout:
-                pass
-            except KeyboardInterrupt:
-                self.stop()
-        self.sock.close()
-
-    def register_handler(self, add):
-        with self.num_handlers_lock:
+                        sys.stdout.write(' server:  new connection from '
+                                         + str(connaddr) + '\n')
+                    handler = self.ConnectionHandler(self, newconn)
+                    handler.start()
+            except socket.timeout:
+                pass
+            except ssl.SSLError:
+                pass
+            except KeyboardInterrupt:
+                self.stop()
+        self.sock.close()
+
+    def register_handler(self, add):
+        with self.num_handlers_lock:
             if add:
                 self.num_handlers += 1
             else: