init: Add more diagnostics for b/223076262.

This adds three more diagnostics to stuck exec services:

1. /proc/pid/fds is dumped
2. /proc/pid/status is dumped
3. HandleSignalFd is called to see if a SIGCHLD got stuck somewhere

Bug: 223076262
Test: while (1) in linkerconfig
Ignore-AOSP-First: diagnostics
Change-Id: Ida601d86e18be9d49b143fb88b418cbc171ecac6