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/basic/socket-util.c

@@ -818,10 +818,7 @@ ssize_t send_one_fd_iov_sa(
                 const struct sockaddr *sa, socklen_t len,
                 int flags) {
 
-        union {
-                struct cmsghdr cmsghdr;
-                uint8_t buf[CMSG_SPACE(sizeof(int))];
-        } control = {};
+        CMSG_BUFFER_TYPE(CMSG_SPACE(sizeof(int))) control = {};
         struct msghdr mh = {
                 .msg_name = (struct sockaddr*) sa,
                 .msg_namelen = len,
@@ -875,10 +872,7 @@ ssize_t receive_one_fd_iov(
                 int flags,
                 int *ret_fd) {
 
-        union {
-                struct cmsghdr cmsghdr;
-                uint8_t buf[CMSG_SPACE(sizeof(int))];
-        } control = {};
+        CMSG_BUFFER_TYPE(CMSG_SPACE(sizeof(int))) control;
         struct msghdr mh = {
                 .msg_control = &control,
                 .msg_controllen = sizeof(control),