restart_wrapper: Be explicit about what signals are forwarded to entr

[jirwin]

Background

In #586 support for forwarding all signals sent to PID 1 on to entr was added. However, it doesn’t fully address the goal of #577, since the developer’s "true" entrypoint isn’t receiving those forwarded signals—only entr is. While this generally works for shutting down child processes, it can also forward unintended signals that break entr.

A concrete example is when the child process managed by entr becomes a zombie and gets re-parented to PID 1. The kernel then sends SIGCHLD to PID 1, which in turn forwards it to entr. These "spurious" SIGCHLD signals can cause entr to stop monitoring or restarting the process, breaking the restart_wrapper functionality.

How to Reproduce

1. kubectl exec into a pod that uses restart_wrapper.
2. Run kill -SIGCHLD <entr-pid>.
3. After this, Tilt will detect file changes and run live update logic, but the process never actually restarts.

Fix

To keep the spirit of #586 this PR changes the code so that only SIGKILL, SIGINT, and SIGTERM are forwarded. This prevents unintended signals (like SIGCHLD) from being passed to entr, avoiding the issue where entr stops managing its child processes after receiving unexpected signals.