Cli version - environment variable edition

[rjbou]

New opam environment variable handling. They are defined per lib, and defined and loaded only at opam client start (no more on library loading). There is no more direct access, each environment variable have its associated field in its lib config, or can be accessed throw OpamXxxConfig.Env.<var> function.
When defined in OpamArg, the cli version is required, to handle cli versioning of environment variable in a slightly different way than options: if not defined in current cli, a warning is printed and it is ignored.

[rjbou]

Remaining:

• preinit env vars in commands like OPAMSELFUPGRADE
• preinit env vars cli main OPAMYES, OPAMCLI
• admin command env vars (OPAMNOAGGREGATE)
• add missing env vars doc
• specify env vars deprecation (except build doc/test) other PRs
• specify new en vars
• specialise env var cli warning messages?
• add test
• externelise Mk fromOpamArg
• Some renaming

[AltGr]

This is pretty great! And finally we can have the documentation of the environment variables besides them :)
Two small nitpicks:

• remember to update the doc index.html when you add modules
• in [WARNING] OPAMNODEPEXTS was added in version 2.1 of the opam CLI, but version 2.0 has been requested, which is older. it is not completely clear wether the variable was used nonetheless or was ignored. Maybe OPAMNODEPEXTS was ignored because CLI v. 2.0 was requested and it was introduced in v.2.1 ?