qrexec: make sure that all the pipes/sockets are closed on cleanup

This will ensure that the child process will receive info that the
connection is closed. Otherwise it could hang on write() or in some
cases read() - on its stdin/stdout.

Thanks @adrelanos for help with debugging.

qrexec/qrexec-agent-data.c

@@ -403,6 +403,26 @@ int process_child_io(libvchan_t *data_vchan,
             }
         }
     }
+    /* make sure that all the pipes/sockets are closed, so the child process
+     * (if any) will know that the connection is terminated */
+    if (stdout_fd != -1) {
+        if (shutdown(stdout_fd, SHUT_RD) < 0) {
+            if (errno == ENOTSOCK)
+                close(stdout_fd);
+        }
+        stdout_fd = -1;
+    }
+    if (stdin_fd != -1) {
+        if (shutdown(stdin_fd, SHUT_WR) < 0) {
+            if (errno == ENOTSOCK)
+                close(stdin_fd);
+        }
+        stdin_fd = -1;
+    }
+    if (stderr_fd != -1) {
+        close(stderr_fd);
+        stderr_fd = -1;
+    }
     return child_process_status;
 }