[patch] RabbitMQ 3.8.9 + OTP23 update, help wanted

[Dave Cottlehuber]

I've got this 95% of the way now, but a fresh pair of eyes is welcomed!

- https://reviews.freebsd.org/D27532

RabbitMQ doesn't support OTP21 anymore, so we need to update which erlang version we use. The default erlang on FreeBSD was OTP21 also, so we are running into a few bugs that "just worked" before because the default OTP was the same as the OTP we run in rabbitmq.

Issues so far:

- startup/shutdown doesn't behave properly
- management plugin can't be enabled

I think this comes from rabbitmq src dropping the ${ERL_DIR} env vars in multiple places, and previously this "just worked", and I haven't found all the places this needs to be fixed yet.

A+
Dave