siphash24: change result argument to uint64_t

Change the "out" parameter from uint8_t[8] to uint64_t. On architectures which
enforce pointer alignment this fixes crashes when we previously cast an
unaligned array to uint64_t*, and on others this should at least improve
performance as the compiler now aligns these properly.

This also simplifies the code in most cases by getting rid of typecasts. The
only place which we can't change is struct duid's en.id, as that is _packed_
and public API, so we can't enforce alignment of the "id" field and have to
use memcpy instead.

src/libsystemd-network/network-internal.c

@@ -56,7 +56,7 @@ const char *net_get_name(struct udev_device *device) {
 
 #define HASH_KEY SD_ID128_MAKE(d3,1e,48,fa,90,fe,4b,4c,9d,af,d5,d7,a1,b1,2e,8a)
 
-int net_get_unique_predictable_data(struct udev_device *device, uint8_t result[8]) {
+int net_get_unique_predictable_data(struct udev_device *device, uint64_t *result) {
         size_t l, sz = 0;
         const char *name = NULL;
         int r;