Automatic merge from submit-queue (batch tested with PRs 39446, 40023, 36853)

Add SIGCHLD handler to pause container

**What this PR does / why we need it**: This allows pause to reap orphaned zombies in a shared PID namespace. (#1615)

**Special notes for your reviewer**: I plan to discuss this with SIG Node to ensure compatibility with future runtimes.

**Release note**: This will have no effect until shared PID namespace is enabled, so recommend release-note-none.

This allows pause to reap zombies in the upcoming Shared PID namespace
(#1615). Uses the better defined sigaction() instead of signal() for all
signals both for consistency (SIGCHLD handler avoids SA_RESTART) and to
avoid the implicit signal()->sigaction() translation of various libc
versions.

Also makes warnings errors and includes a tool to make orphaned zombies
for manual testing.