in-addr-util: add byte accessor array to union in_addr_union

It's pretty useful to be able to access the bytes generically, without
acknowledging a specific family, hence let's a third way to access an
in_addr_union.

src/basic/in-addr-util.h

@@ -12,6 +12,7 @@
 union in_addr_union {
         struct in_addr in;
         struct in6_addr in6;
+        uint8_t bytes[CONST_MAX(sizeof(struct in_addr), sizeof(struct in6_addr))];
 };
 
 struct in_addr_data {