Refactor DTLSSocket to use Socket::getpeername()

Also, let DTLSSocket to be a friend of InternetSocket so it can do
the name resolution from its _stack.

+ Some whitespace fixes

features/netsocket/DTLSSocket.cpp

@@ -25,19 +25,27 @@
 
 nsapi_error_t DTLSSocket::connect(const char *host, uint16_t port)
 {
-    if (!_remote_address) {
-        nsapi_error_t err = _stack->gethostbyname(host, &_remote_address);
+    SocketAddress addr;
+    nsapi_error_t ret;
+
+    ret = _udp_socket.getpeername(&addr);
+    if (ret != NSAPI_ERROR_NO_CONNECTION) {
+        return ret;
+    }
+
+    if (!addr || ret == NSAPI_ERROR_NO_CONNECTION) {
+        nsapi_error_t err = _udp_socket._stack->gethostbyname(host, &addr);
         if (err) {
             return NSAPI_ERROR_DNS_FAILURE;
         }
 
-        _remote_address.set_port(port);
+        addr.set_port(port);
 
         set_hostname(host);
-        _udp_socket.connect(_remote_address); // UDPSocket::connect() cannot fail
+        _udp_socket.connect(addr); // UDPSocket::connect() cannot fail
     }
 
-    return connect(_remote_address);
+    return connect(addr);
 }
 
 DTLSSocket::~DTLSSocket()

features/netsocket/DTLSSocket.h

@@ -1,3 +1,20 @@
+/*
+ * Copyright (c) 2018 ARM Limited
+ * SPDX-License-Identifier: Apache-2.0
+ *
+ * Licensed under the Apache License, Version 2.0 (the "License");
+ * you may not use this file except in compliance with the License.
+ * You may obtain a copy of the License at
+ *
+ *     http://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing, software
+ * distributed under the License is distributed on an "AS IS" BASIS,
+ * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+ * See the License for the specific language governing permissions and
+ * limitations under the License.
+ */
+
 #ifndef DTLSSOCKET_H
 #define DTLSSOCKET_H
 
@@ -46,7 +63,6 @@ public:
      *  @return         0 on success, negative error code on failure
      */
     virtual nsapi_error_t open(NetworkStack *stack) {
-        _stack = stack;
         return _udp_socket.open(stack);
     }
 
@@ -70,8 +86,6 @@ public:
 
 private:
     UDPSocket _udp_socket;
-    NetworkStack *_stack;
-    SocketAddress _remote_address;
 };
 
 #endif

features/netsocket/InternetSocket.h

@@ -175,6 +175,8 @@ protected:
     static const int WRITE_FLAG    = 0x2u;
     static const int FINISHED_FLAG = 0x3u;
 
+    friend class DTLSSocket;  // Allow DTLSSocket::connect() to do name resolution on the _stack
+
 #endif //!defined(DOXYGEN_ONLY)
 };