WIP: Patch release 7.3 for freebsd

Context

This merge request ports v7-release to FreeBSD.

Some of the changes should be incorporated in a more dune-friendly fashion. I need help with that from someone more expert in dune.

This merge request operates against v7-release.

Related: #494 (closed), #764 (closed), #894 (closed) Fixes: #769 (closed)

Manually testing the MR

Builds on FreeBSD 12.1.

Install packages: opam, pkgconf, hidapi, libev, gmp, lmdb, gmake

From the top-level directory:

  OPAMROOT=./_root
  export OPAMROOT
  opam init --bare --no-setup --no-opamrc
  gmake build-deps
  eval $(opam env)
  gmake all

Automatic testing

No relevant changes

Reviewers

@smondet