Revert "Revert "Implement getifaddrs(3)/freeifaddrs(3).""

This reverts commit 76814a8250.

This differs from the original in fixing the GCC -Werror build:

  bionic/libc/bionic/ifaddrs.cpp: In function 'void __handle_netlink_response(ifaddrs**, nlmsghdr*)':
  bionic/libc/bionic/ifaddrs.cpp:113:62: error: use of old-style cast [-Werror=old-style-cast]
       ifinfomsg* ifi = reinterpret_cast<ifinfomsg*>(NLMSG_DATA(hdr));

This appears to be a GCC bug; the GCC command-line correctly uses -isystem,
and manually adding #pragma GCC system_header doesn't help. So just turn the
warning off for GCC for now. We won't need to worry about building with GCC
soon anyway.

Bug: http://b/26238832
Change-Id: I01615bd335edf11baf487b1c83a9157cd780f4a1

tests/Android.mk

@@ -62,6 +62,7 @@ libBionicStandardTests_src_files := \
     ftw_test.cpp \
     getauxval_test.cpp \
     getcwd_test.cpp \
+    ifaddrs_test.cpp \
     inttypes_test.cpp \
     libc_logging_test.cpp \
     libgen_basename_test.cpp \

tests/ifaddrs_test.cpp

@@ -0,0 +1,38 @@
+/*
+ * Copyright (C) 2015 The Android Open Source Project
+ *
+ * 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.
+ */
+
+#include <gtest/gtest.h>
+
+#include <ifaddrs.h>
+
+TEST(ifaddrs, freeifaddrs_null) {
+  freeifaddrs(nullptr);
+}
+
+TEST(ifaddrs, getifaddrs_smoke) {
+  ifaddrs* addrs = nullptr;
+
+  ASSERT_EQ(0, getifaddrs(&addrs));
+  ASSERT_TRUE(addrs != nullptr);
+
+  bool saw_loopback = false;
+  for (ifaddrs* addr = addrs; addr != nullptr; addr = addr->ifa_next) {
+    if (addr->ifa_name && strcmp(addr->ifa_name, "lo") == 0) saw_loopback = true;
+  }
+  ASSERT_TRUE(saw_loopback);
+
+  freeifaddrs(addrs);
+}