Add a case-insensitive option directly to siphash 2-4 implementation

Formerly, the isc_hash32() would have to change the key in a local copy
to make it case insensitive.  Change the isc_siphash24() and
isc_halfsiphash24() functions to lowercase the input directly when
reading it from the memory and converting the uint8_t * array to
64-bit (respectively 32-bit numbers).

lib/ns/client.c

@@ -1196,7 +1196,7 @@ compute_cookie(ns_client_t *client, uint32_t when, uint32_t nonce,
 			UNREACHABLE();
 		}
 
-		isc_siphash24(secret, input, inputlen, digest);
+		isc_siphash24(secret, input, inputlen, true, digest);
 		isc_buffer_putmem(buf, digest, 8);
 		break;
 	}