The main changes are:
- Adapt to libsupport.
- Synchronize the signal handler using atomics.
- Replace waitpid by waitid calls.
- Use support_process_state_wait to wait for child state.
- Add tests for P_PGID and P_ALL.
- Use sigwaitinfo instead of global state set by the signal handler.
Checked on x86_64-linux-gnu and i686-linux-gnu.