tree-wide: use recvmsg_safe() at various places

Let's be extra careful whenever we return from recvmsg() and see
MSG_CTRUNC set. This generally means we ran into a programming error, as
we didn't size the control buffer large enough. It's an error condition
we should at least log about, or propagate up. Hence do that.

This is particularly important when receiving fds, since for those the
control data can be of any size. In particular on stream sockets that's
nasty, because if we miss an fd because of control data truncation we
cannot recover, we might not even realize that we are one off.

(Also, when failing early, if there's any chance the socket might be
AF_UNIX let's close all received fds, all the time. We got this right
most of the time, but there were a few cases missing. God, UNIX is hard
to use)

src/basic/socket-util.c

@@ -901,9 +901,9 @@ ssize_t receive_one_fd_iov(
          * combination with send_one_fd().
          */
 
-        k = recvmsg(transport_fd, &mh, MSG_CMSG_CLOEXEC | flags);
+        k = recvmsg_safe(transport_fd, &mh, MSG_CMSG_CLOEXEC | flags);
         if (k < 0)
-                return (ssize_t) -errno;
+                return k;
 
         CMSG_FOREACH(cmsg, &mh) {
                 if (cmsg->cmsg_level == SOL_SOCKET &&
@@ -915,12 +915,13 @@ ssize_t receive_one_fd_iov(
                 }
         }
 
-        if (!found)
+        if (!found) {
                 cmsg_close_all(&mh);
 
-        /* If didn't receive an FD or any data, return an error. */
-        if (k == 0 && !found)
-                return -EIO;
+                /* If didn't receive an FD or any data, return an error. */
+                if (k == 0)
+                        return -EIO;
+        }
 
         if (found)
                 *ret_fd = *(int*) CMSG_DATA(found);