Add v4tov6.

util.c

@@ -284,3 +284,10 @@ v4mapped(const unsigned char *address)
 {
     return in_prefix(address, v4prefix, 96);
 }
+
+void
+v4tov6(unsigned char *dst, const unsigned char *src)
+{
+    memcpy(dst, v4prefix, 12);
+    memcpy(dst + 12, src, 4);
+}

util.h

@@ -50,6 +50,7 @@ int parse_net(const char *net, unsigned char *prefix_r, unsigned char *plen_r);
 int wait_for_fd(int direction, int fd, int msecs);
 int martian_prefix(const unsigned char *prefix, int plen);
 int v4mapped(const unsigned char *address);
+void v4tov6(unsigned char *dst, const unsigned char *src);
 
 /* If debugging is disabled, we want to avoid calling format_address
    for every omitted debugging message.  So debug is a macro.  But