kernel-netlink: Define netlink buffer as an union having a netlink header

This allows us to streamline the netlink buffers, and avoid extensive
casting.

src/libhydra/plugins/kernel_netlink/kernel_netlink_shared.h

@@ -26,7 +26,10 @@
  * 1024 byte is currently sufficient for all operations. Some platform
  * require an enforced aligment to four bytes (e.g. ARM).
  */
-typedef u_char netlink_buf_t[1024] __attribute__((aligned(RTA_ALIGNTO)));
+typedef union {
+	struct nlmsghdr hdr;
+	u_char bytes[1024];
+} netlink_buf_t __attribute__((aligned(RTA_ALIGNTO)));
 
 typedef struct netlink_socket_t netlink_socket_t;