shared: make sure foo.bar and foobar result in different domain name hashes

This also introduces a new macro siphash24_compress_byte() which is useful to add a single byte into the hash stream,
and ports one user over to it.

src/basic/siphash24.h

@@ -16,6 +16,8 @@ struct siphash {
 
 void siphash24_init(struct siphash *state, const uint8_t k[16]);
 void siphash24_compress(const void *in, size_t inlen, struct siphash *state);
+#define siphash24_compress_byte(byte, state) siphash24_compress((const uint8_t[]) { (byte) }, 1, (state))
+
 uint64_t siphash24_finalize(struct siphash *state);
 
 uint64_t siphash24(const void *in, size_t inlen, const uint8_t k[16]);