macos: can potentially avoid needing opam setenv

[avsm]

I recently discovered via @djs55 that macOS has a launchctl setenv command that can set the environment variables for a user without having to be root.

There is more information here: http://www.dowdandassociates.com/blog/content/howto-set-an-environment-variable-in-mac-os-x-launchd-plist/

opam could possibly generate a launchctl file that sets the relevant environment variables and avoid the need for a user to have to run the env setting every time.

[AltGr]

Interesting indeed, sounds better than our current (new) approach of using shell hooks to automatically update. Won't be in 2.0.0 but definitely worth investigating ; if I understand correctly, it would mean writing a ~/Library/LaunchAgents/opam-env file on every opam switch command (or whenever we rewrite ~/.opam/opam-init/variables.*sh)