tree-wide: make sure our control buffers are properly aligned

We always need to make them unions with a "struct cmsghdr" in them, so
that things properly aligned. Otherwise we might end up at an unaligned
address and the counting goes all wrong, possibly making the kernel
refuse our buffers.

Also, let's make sure we initialize the control buffers to zero when
sending, but leave them uninitialized when reading.

Both the alignment and the initialization thing is mentioned in the
cmsg(3) man page.

src/nspawn/nspawn.c

@@ -3687,11 +3687,8 @@ static int nspawn_dispatch_notify_fd(sd_event_source *source, int fd, uint32_t r
                 .iov_base = buf,
                 .iov_len = sizeof(buf)-1,
         };
-        union {
-                struct cmsghdr cmsghdr;
-                uint8_t buf[CMSG_SPACE(sizeof(struct ucred)) +
-                            CMSG_SPACE(sizeof(int) * NOTIFY_FD_MAX)];
-        } control = {};
+        CMSG_BUFFER_TYPE(CMSG_SPACE(sizeof(struct ucred)) +
+                         CMSG_SPACE(sizeof(int) * NOTIFY_FD_MAX)) control;
         struct msghdr msghdr = {
                 .msg_iov = &iovec,
                 .msg_iovlen = 1,