devpts: Check ISIG flag before generating signals for VINTR/VSUSP/VQUIT

[tanyifeng]

Signal characters (Ctrl-C/Z/) unconditionally generated signals regardless of the ISIG flag. Linux only generates these signals when L_ISIG(tty) is true (n_tty.c:n_tty_receive_char_special).

Also fix signal characters leaking into the read buffer — Linux consumes them in n_tty_receive_signal_char() and never delivers them to the reading process.

[tanyifeng]

The babi text editor's test suite reproduces this issue out of the box. Its [tmux]-parameterized tests in tests/features/replace_test.py send Ctrl-\ (0x1c) through a real PTY via tmux. Without the fix, gVisor unconditionally generates SIGQUIT for Ctrl-\ regardless of the ISIG setting, killing the tmux session.

Run a single test case to reproduce quickly:

docker run --rm --runtime=runsc python:3.11-slim bash -c "
  apt-get update -qq && apt-get install -y -qq tmux git > /dev/null 2>&1 &&
  pip install -q pytest hecate coverage covdefaults &&
  git clone --depth 1 https://github.com/asottile/babi.git /tmp/babi &&
  cd /tmp/babi && pip install -e . -q &&
  python -m pytest tests/features/replace_test.py -k 'test_replace_cancel[tmux-Enter]' --no-header -rN --tb=line
"

Before fix — FAILED (Ctrl-\ generates SIGQUIT, killing tmux server):

E   subprocess.CalledProcessError: Command '['tmux', '-u', '-L', '51fd4b4d2a5022ff', 'list-buffers']' returned non-zero exit status 1.
================= 1 failed, 51 deselected, 1 warning in 5.42s ==================

After fix — PASSED (Ctrl-\ passed as data byte 0x1c to babi):

================= 1 passed, 51 deselected, 1 warning in 4.71s ==================

[ayushr2]

Thanks for the fix!