Large refactor of EDNS0 UDP packet size handling.

This was kinda strange before, with a lot of cargo-cult copied code,
and no clear strategy.

Now it works like this:

When talking upstream we always add a pseudoheader, and set the
UDP packet size to --edns-packet-max unless we've had problems
talking to a server, when it's reduced to 1280 if that fixes things.

Answering queries from downstream, we get the answer (either from
upstream or local data) If local data won't fit the advertised size
(or 512 if there's not pseudoheader) return truncated. If upstream
returns truncated, do likewise. If upstream is OK, but the answer is
too big for downstream, truncate the answer.

src/dns-protocol.h

@@ -142,15 +142,15 @@ struct dns_header {
 #define RCODE(x)           ((x)->hb4 & HB4_RCODE)
 #define SET_RCODE(x, code) (x)->hb4 = ((x)->hb4 & ~HB4_RCODE) | code
   
-#define GETSHORT(s, cp) { \
+#define GETSHORT(s, cp) do { \
 	unsigned char *t_cp = (unsigned char *)(cp); \
 	(s) = ((u16)t_cp[0] << 8) \
 	    | ((u16)t_cp[1]) \
 	    ; \
 	(cp) += 2; \
-}
+  } while(0)
 
-#define GETLONG(l, cp) { \
+#define GETLONG(l, cp) do { \
 	unsigned char *t_cp = (unsigned char *)(cp); \
 	(l) = ((u32)t_cp[0] << 24) \
 	    | ((u32)t_cp[1] << 16) \
@@ -158,17 +158,17 @@ struct dns_header {
 	    | ((u32)t_cp[3]) \
 	    ; \
 	(cp) += 4; \
-}
+  } while (0)
 
-#define PUTSHORT(s, cp) { \
+#define PUTSHORT(s, cp) do { \
 	u16 t_s = (u16)(s); \
 	unsigned char *t_cp = (unsigned char *)(cp); \
 	*t_cp++ = t_s >> 8; \
 	*t_cp   = t_s; \
 	(cp) += 2; \
-}
+  } while(0)
 
-#define PUTLONG(l, cp) { \
+#define PUTLONG(l, cp) do { \
 	u32 t_l = (u32)(l); \
 	unsigned char *t_cp = (unsigned char *)(cp); \
 	*t_cp++ = t_l >> 24; \
@@ -176,7 +176,7 @@ struct dns_header {
 	*t_cp++ = t_l >> 8; \
 	*t_cp   = t_l; \
 	(cp) += 4; \
-}
+  } while (0)
 
 #define CHECK_LEN(header, pp, plen, len) \
     ((size_t)((pp) - (unsigned char *)(header) + (len)) <= (plen))