utils: Ensure that the buffer for struct cmsghdr is suitably-aligned

A char array on the stack is not guaranteed to have any particular
alignment.

Resolves: #637
Signed-off-by: Simon McVittie <smcv@collabora.com>

utils.c

@@ -758,13 +758,16 @@ send_pid_on_socket (int sockfd)
   struct msghdr msg = {};
   struct iovec iov = { buf, sizeof (buf) };
   const ssize_t control_len_snd = CMSG_SPACE(sizeof(struct ucred));
-  char control_buf_snd[control_len_snd];
+  union {
+      char buf[control_len_snd];
+      struct cmsghdr align;
+  } control_buf_snd;
   struct cmsghdr *cmsg;
   struct ucred cred;
 
   msg.msg_iov = &iov;
   msg.msg_iovlen = 1;
-  msg.msg_control = control_buf_snd;
+  msg.msg_control = control_buf_snd.buf;
   msg.msg_controllen = control_len_snd;
 
   cmsg = CMSG_FIRSTHDR(&msg);
@@ -800,12 +803,15 @@ read_pid_from_socket (int sockfd)
   struct msghdr msg = {};
   struct iovec iov = { recv_buf, sizeof (recv_buf) };
   const ssize_t control_len_rcv = CMSG_SPACE(sizeof(struct ucred));
-  char control_buf_rcv[control_len_rcv];
+  union {
+      char buf[control_len_rcv];
+      struct cmsghdr align;
+  } control_buf_rcv;
   struct cmsghdr* cmsg;
 
   msg.msg_iov = &iov;
   msg.msg_iovlen = 1;
-  msg.msg_control = control_buf_rcv;
+  msg.msg_control = control_buf_rcv.buf;
   msg.msg_controllen = control_len_rcv;
 
   if (TEMP_FAILURE_RETRY (recvmsg (sockfd, &msg, 0)) < 0)