Implement "DNS-0x20 encoding".

This provides extra protection against reply-spoof attacks.

Since DNS queries are case-insensitive, it's possible to randomly flip
the case of letters in a query and still get the correct answer back.
This adds an extra dimension for a cache-poisoning attacker to guess
when sending replies in-the-blind since it's expected that the
legitimate answer will have the same pattern of upper and lower case
as the query, so any replies which don't can be ignored as
malicious.

The amount of extra entropy clearly depends on the number
of a-z and A-Z characters in the query, and this implementation puts a
hard limit of 32 bits to make rescource allocation easy. This about
doubles entropy over the standard random ID and random port
combination.

src/auth.c

@@ -137,7 +137,7 @@ size_t answer_auth(struct dns_header *header, char *limit, size_t qlen, time_t n
       nameoffset = p - (unsigned char *)header;
 
       /* now extract name as .-concatenated string into name */
-      if (!extract_name(header, qlen, &p, name, 1, 4))
+      if (!extract_name(header, qlen, &p, name, EXTR_NAME_EXTRACT, 4))
 	return 0; /* bad packet */
  
       GETSHORT(qtype, p);