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

This reverts commit 76814a8250fd5e1502909ef5e10e4d166cc96c0e.

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
diff --git a/libc/include/ifaddrs.h b/libc/include/ifaddrs.h
new file mode 100644
index 0000000..54a5a2c
--- /dev/null
+++ b/libc/include/ifaddrs.h
@@ -0,0 +1,59 @@
+/*
+ * Copyright (C) 2015 The Android Open Source Project
+ * All rights reserved.
+ *
+ * Redistribution and use in source and binary forms, with or without
+ * modification, are permitted provided that the following conditions
+ * are met:
+ *  * Redistributions of source code must retain the above copyright
+ *    notice, this list of conditions and the following disclaimer.
+ *  * Redistributions in binary form must reproduce the above copyright
+ *    notice, this list of conditions and the following disclaimer in
+ *    the documentation and/or other materials provided with the
+ *    distribution.
+ *
+ * THIS SOFTWARE IS PROVIDED BY THE COPYRIGHT HOLDERS AND CONTRIBUTORS
+ * "AS IS" AND ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT
+ * LIMITED TO, THE IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS
+ * FOR A PARTICULAR PURPOSE ARE DISCLAIMED. IN NO EVENT SHALL THE
+ * COPYRIGHT OWNER OR CONTRIBUTORS BE LIABLE FOR ANY DIRECT, INDIRECT,
+ * INCIDENTAL, SPECIAL, EXEMPLARY, OR CONSEQUENTIAL DAMAGES (INCLUDING,
+ * BUT NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS OR SERVICES; LOSS
+ * OF USE, DATA, OR PROFITS; OR BUSINESS INTERRUPTION) HOWEVER CAUSED
+ * AND ON ANY THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT LIABILITY,
+ * OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY OUT
+ * OF THE USE OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF
+ * SUCH DAMAGE.
+ */
+
+#ifndef _IFADDRS_H_
+#define _IFADDRS_H_
+
+#include <sys/cdefs.h>
+#include <netinet/in.h>
+#include <sys/socket.h>
+
+__BEGIN_DECLS
+
+struct ifaddrs {
+  struct ifaddrs* ifa_next;
+  char* ifa_name;
+  unsigned int ifa_flags;
+  struct sockaddr* ifa_addr;
+  struct sockaddr* ifa_netmask;
+  union {
+    struct sockaddr* ifu_broadaddr;
+    struct sockaddr* ifu_dstaddr;
+  } ifa_ifu;
+  void* ifa_data;
+};
+
+#define ifa_broadaddr ifa_ifu.ifu_broadaddr
+#define ifa_dstaddr ifa_ifu.ifu_dstaddr
+
+void freeifaddrs(struct ifaddrs*);
+int getifaddrs(struct ifaddrs**);
+
+__END_DECLS
+
+#endif