init: use signalfd to catch SIGCHLD

Previously, if init received too many SIGCHLD signals, then the write to
signal_write_fd could fail with EAGAIN. The handler tried to log the
EAGAIN error, and init deadlocked if the interrupted init process had
already acquired a logging-related lock.

Bug: b/77867680
Test: manual
Change-Id: Ief0b5e94d8517827a5a7d03773391ba3ba9447c4