Implement getifaddrs(3)/freeifaddrs(3).

Time to dust off the old libcore implementation from gingerbread and add it
to bionic. Unlike the original, this actually looks at both RTM_NEWLINK and
RTM_NEWADDR.

Bug: http://b/26238832
Change-Id: I7bb4b432deb766065b66b9c9ff36ed68249aba82
diff --git a/libc/libc.x86.map b/libc/libc.x86.map
index 7120e7a..a4b4494 100644
--- a/libc/libc.x86.map
+++ b/libc/libc.x86.map
@@ -1272,8 +1272,10 @@
     __pwrite64_chk;
     __write_chk;
     fileno_unlocked;
+    freeifaddrs;
     getgrgid_r;
     getgrnam_r;
+    getifaddrs;
     preadv;
     preadv64;
     prlimit; # arm mips x86