MINOR: debug: add random delay injection with "debug dev delay-inj"

MINOR: debug: add random delay injection with "debug dev delay-inj"

The goal is to send signals to random threads at random instants so that
they spin for a random delay in a relax() loop, trying to give back the
CPU to another competing hardware thread, in hope that from time to time
this can trigger in critical areas and increase the chances to provoke a
latent concurrency bug. For now none were observed.

For example, this command starts 64 such tasks waking after random delays
of 0-1ms and delivering signals to trigger such loops on 3 random threads:

  for i in {1..64}; do
    socat - /tmp/sock1 <<< "expert-mode on;debug dev delay-inj 2 3"
  done

This command is only enabled when DEBUG_DEV is set at build time.