MINOR: debug: report in port_mortem whether a container was detected

MINOR: debug: report in port_mortem whether a container was detected

Containers often cause significant trouble depending on how they're
set up, and they're not always trivial for their users to extract info
from. Here we're trying to detect if we're running inside a container
on Linux. There are plenty of approaches and none is perfectly clean
nor reliable, which makes sense since the goal is to remain transparent
enough.

One interesting approach is to rely on the observation that containers
generally do not expose most kernel threads, and that the very firsts
of them are extremely stable across all kernel versions: pid 2 was
called "keventd" in kernel 2.4, became "kthreadd" in kernel 2.6, and
has since not changed. This is true on all architectures tested, even
with highly stripped down kernels such as those found on 15 year-old
OpenWRT images. And this one doesn't appear inside containers. Thus
here we check if we find such a thread via /proc and whether it's
called keventd or kthreadd, to detect a container, and we set the
"cont_techno" variable to "yes" or "no" depending on what is found.