[followup #433] MirageOS 3 working branch

[avsm]

Also includes opam fixes in #457

[avsm]

Added a dependency on irmin-http due to

- ocamlfind ocamldep -package 'asl win-eventlog cmdliner mtime.os logs.cli' -package 'protocol-9p.unix datakit-server.fs9p' -package 'hvsock.lwt-unix named-pipe.lwt uri mirage-flow' -package 'threads conduit.lwt-unix hvsock.lwt' -package 'protocol-9p.unix camlzip' -package 'datakit-server.vfs datakit-server.fs9p' -package 'irmin.http cohttp.lwt irmin-watcher' -package 'git irmin irmin.git irmin.mem irmin-watcher' -package 'cmdliner fmt.cli fmt.tty logs.fmt asetmap' -package prometheus-app -package 'conduit.lwt-unix irmin lwt.unix uri camlzip git tc' -package 'bytes lwt astring logs result cstruct fmt rresult' -modules src/datakit/main.ml > src/datakit/main.ml.depends
- + ocamlfind ocamldep -package 'asl win-eventlog cmdliner mtime.os logs.cli' -package 'protocol-9p.unix datakit-server.fs9p' -package 'hvsock.lwt-unix named-pipe.lwt uri mirage-flow' -package 'threads conduit.lwt-unix hvsock.lwt' -package 'protocol-9p.unix camlzip' -package 'datakit-server.vfs datakit-server.fs9p' -package 'irmin.http cohttp.lwt irmin-watcher' -package 'git irmin irmin.git irmin.mem irmin-watcher' -package 'cmdliner fmt.cli fmt.tty logs.fmt asetmap' -package prometheus-app -package 'conduit.lwt-unix irmin lwt.unix uri camlzip git tc' -package 'bytes lwt astring logs result cstruct fmt rresult' -modules src/datakit/main.ml > src/datakit/main.ml.depends
- ocamlfind: Package `irmin.http' not found

https://ci.ocaml.io/log/saved/docker-run-7ecbcd6c56daa1997c335ed5151275e4/12be4d410077b4d67829e2ada6bd6a30de20397b

[talex5]

Looks like this is blocked on:

• hvsock support for Mirage 3 ([WIP] mirage-dev branch for MirageOS3 mirage/ocaml-hvsock#41)
• Irmin 1.0 support (0.12.0 conflicts with Mirage 3)

[samoht]

Closing as it is superseded by #529 and #531