Prefix cmsg_nxthdr with "__".

This brings us on par with glibc.

To avoid breaking clients, temporary keep cmsg_nxthdr until the next NDK
refresh.

Bug: 15822452
Change-Id: I24c24e68c31f4f2b8f3d2df7acd575cb75174173
diff --git a/libc/bionic/cmsg_nxthdr.cpp b/libc/bionic/cmsg_nxthdr.cpp
index 6f0a47c..8a2b33e 100644
--- a/libc/bionic/cmsg_nxthdr.cpp
+++ b/libc/bionic/cmsg_nxthdr.cpp
@@ -28,7 +28,7 @@
 
 #include <sys/socket.h>
 
-cmsghdr* cmsg_nxthdr(msghdr* msg, cmsghdr* cmsg) {
+cmsghdr* __cmsg_nxthdr(msghdr* msg, cmsghdr* cmsg) {
   cmsghdr* ptr;
   ptr = reinterpret_cast<cmsghdr*>(reinterpret_cast<char*>(cmsg) + CMSG_ALIGN(cmsg->cmsg_len));
   size_t len = reinterpret_cast<char*>(ptr+1) - reinterpret_cast<char*>(msg->msg_control);
@@ -37,3 +37,6 @@
   }
   return ptr;
 }
+
+// TODO: remove after NDK refresh.
+__weak_alias(cmsg_nxthdr, __cmsg_nxthdr);