Background node process corrupts terminal state with tcsetattr() on exit

* **Version**: v14.9.0 * **Platform**: NixOS 21.03, Linux 5.8.11 x86_64 * **Subsystem**: `ResetStdio` in `src/node.cc` In Bash, run the following: ```bash node -e 'setTimeout(() => {}, 2000)' & sleep 1 ``` This launches `node -e 'setTimeout(() => {}, 2000)'` in the background and `sleep 1` in the foreground. After 1 second, control returns to Bash. After 2 seconds, Node exits and corrupts Bash’s terminal state: * the ↑, ↓, →, ← arrow keys start printing `^[[A`, `^[[B`, `^[[C`, `^[[D` instead of scrolling through the Bash history or moving the insert point, * the Tab key starts moving the cursor 8 spaces forward instead of Tab-completing the command, * Ctrl+A and Ctrl+E print `^A` and `^E` instead of jumping to the beginning and end of the command, etc. This happens when the `ResetStdio` handler uses `tcsetattr` to “restore” the terminal to the state it was in when Node started (#24260). This interferes with Bash, which uses and expects a different terminal state than `sleep 1`. (The bug is sometimes reproducible without the `sleep 1` depending on whether Node records the initial terminal state before Bash updates it, but the `sleep 1` makes it reliable.) Normally, the terminal state would be protected from such undesired modifications by a background process: the kernel generates `SIGTTOU` to suspend the process until it’s brought to the foreground. But `ResetStdio` now deliberately overrides this protection by blocking `SIGTTOU` (#28535). IMO, both #24260 and #28535 should be reverted. Node is a programming language; it should never make changes to the terminal state that programs did not request.