res_send: use a temp buffer if caller's buffer is under 512 bytes

for extremely small buffer sizes, the DNS query core in __res_msend
may malfunction completely, being unable to get even the headers to
determine the response code. but there is also a problem for
reasonable sizes under 512 bytes: __res_msend is unable to determine
if the udp answer was truncated at the recv layer, in which case it
may be incomplete, and res_send is then unable to honor its contract
to return the length of the full, non-truncated answer.

at present, res_send does not honor that contract anyway when the full
answer would exceed 512 bytes, since there is no tcp fallback, but
this change at least makes it consistent in a context where this is
the only "full answer" to be had.

src/network/res_send.c

@@ -1,8 +1,16 @@
 #include <resolv.h>
+#include <string.h>
 
 int __res_send(const unsigned char *msg, int msglen, unsigned char *answer, int anslen)
 {
-	int r = __res_msend(1, &msg, &msglen, &answer, &anslen, anslen);
+	int r;
+	if (anslen < 512) {
+		unsigned char buf[512];
+		r = __res_send(msg, msglen, buf, sizeof buf);
+		if (r >= 0) memcpy(answer, buf, r < anslen ? r : anslen);
+		return r;
+	}
+	r = __res_msend(1, &msg, &msglen, &answer, &anslen, anslen);
 	return r<0 || !anslen ? -1 : anslen;
 }