system.c: Flush node input after running external programs.

If a child program that was supposed to receive user input didn't
receive it before exiting, then input the user intended for the
program might go back to the BBS afterwards, potentially doing unwanted
things in the menu system. Since any "leftover" input should be
discarded, flush any pending input on the node after the child has exited.

bbs/system.c

@@ -1182,6 +1182,7 @@ static int __bbs_execvpe_fd(struct bbs_node *node, int usenode, int fdin, int fd
 		 */
 		node->childpid = 0;
 		if (usenode) {
+			int buffered = node->buffered;
 			/* Restore original terminal settings.
 			 * This way, if whatever program was executed exited
 			 * without leaving the terminal in a good/usable state,
@@ -1190,6 +1191,16 @@ static int __bbs_execvpe_fd(struct bbs_node *node, int usenode, int fdin, int fd
 			if (tcsetattr(node->slavefd, TCSANOW, &term)) {
 				bbs_error("tcsetattr failed: %s\n", strerror(errno));
 			}
+
+			/* Flush any input that may still be pending when the program exited.
+			 * If there was still input waiting for the program, discard it all,
+			 * or it could erroneously be sent to the BBS when we return,
+			 * wreaking havoc. */
+			bbs_node_unbuffer(node);
+			bbs_node_flush_input(node);
+			if (buffered) {
+				bbs_node_buffer(node); /* Not sure if it's really necessary to restore... but doesn't hurt */
+			}
 		}
 	}