fix: respond to SIGTERM/SIGINT in default init script

[delaidam]

Summary

• update the default init script to trap SIGTERM and SIGINT
• keep the shell as PID 1 while running sleep infinity as a child process
• document PID 1 signal handling behavior for custom init scripts
• add an integration test covering SIGTERM and SIGINT shutdown behavior

Fixes #467

Test Plan

• go build ./...
• go vet ./...
• go test ./options/...
• git diff --check
• golangci-lint run --timeout=10m
• go test $(go list ./... | grep -v '/integration')
• make test-registry
• go test -count=1 -v ./integration/... -run TestDefaultInitScriptRespondsToSignals