init: Add more diagnostics for signalfd hangs.

This adds two new diagnostics. First, signalfd reads are now non-blocking. If the read takes more than 10 seconds, we log an error.

Second, init now wakes up from epoll() every 10 seconds. If it waits on an "exec" command for more than 10 seconds, it logs an error.

This change will be reverted as soon as we get feedback.

Bug: 223076262
Test: device boots
Change-Id: I7ee98d159599217a641b3de2564a92c2435f57ef