Implement martian_prefix.

util.c

@@ -201,3 +201,12 @@ wait_for_fd(int direction, int fd, int msecs)
 
     return rc;
 }
+
+int
+martian_prefix(const unsigned char *prefix, int plen)
+{
+    return
+        (plen >= 8 && prefix[0] == 0xFF) ||
+        (plen >= 10 && prefix[0] == 0xFE && (prefix[1] & 0xC0) == 0x80) ||
+        (plen >= 128 && memcmp(prefix, zeroes, 16) == 0);
+}

util.h

@@ -37,6 +37,7 @@ const char *format_address(const unsigned char *address);
 int parse_address(const char *address, unsigned char *addr_r);
 int parse_net(const char *net, unsigned char *prefix_r, unsigned short *plen_r);
 int wait_for_fd(int direction, int fd, int msecs);
+int martian_prefix(const unsigned char *prefix, int plen);
 
 /* If debugging is disabled, we want to avoid calling format_address
    for every omitted debugging message.  So debug is a macro.  But