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);
diff --git a/libc/include/sys/socket.h b/libc/include/sys/socket.h
index 7edaac9..ae2f238 100644
--- a/libc/include/sys/socket.h
+++ b/libc/include/sys/socket.h
@@ -107,7 +107,7 @@
   int cmsg_type;
 };
 
-#define CMSG_NXTHDR(mhdr, cmsg) cmsg_nxthdr((mhdr), (cmsg))
+#define CMSG_NXTHDR(mhdr, cmsg) __cmsg_nxthdr((mhdr), (cmsg))
 #define CMSG_ALIGN(len) ( ((len)+sizeof(long)-1) & ~(sizeof(long)-1) )
 #define CMSG_DATA(cmsg) ((void*)((char*)(cmsg) + CMSG_ALIGN(sizeof(struct cmsghdr))))
 #define CMSG_SPACE(len) (CMSG_ALIGN(sizeof(struct cmsghdr)) + CMSG_ALIGN(len))
@@ -117,7 +117,7 @@
    ? (struct cmsghdr*) (msg)->msg_control : (struct cmsghdr*) NULL)
 #define CMSG_OK(mhdr, cmsg) ((cmsg)->cmsg_len >= sizeof(struct cmsghdr) &&   (cmsg)->cmsg_len <= (unsigned long)   ((mhdr)->msg_controllen -   ((char*)(cmsg) - (char*)(mhdr)->msg_control)))
 
-struct cmsghdr* cmsg_nxthdr(struct msghdr*, struct cmsghdr*);
+struct cmsghdr* __cmsg_nxthdr(struct msghdr*, struct cmsghdr*);
 
 #define SCM_RIGHTS 0x01
 #define SCM_CREDENTIALS 0x02