Improve the CLI help

[dduportal]

What does this PR do?

This PR introduces the following changes:

• For end-users: It improves the clarity of the traefik --help command line (CLI) output
• For maintainers and contributors: Update of the dependency Flaeg to the version 1.3.0
• For maintainers and contributors: It allows the commands make dep-ensure and make dep-prune to be run on macOS and BSD

Motivation

The current state of the traefik --help is very confusing: if you are getting started, you don't know where to put commands: before, after flags?

When updating dependencies on my mac, I also was facing an error message when running make dep-ensure. it was a simple fix so it has been smoothly added into the scope of this PR.

More

• [-] Added/updated tests
• [-] Added/updated documentation

Additional Notes

Example of the new output:

$ ./traefik --help
traefik is a modern HTTP reverse proxy and load balancer made to deploy microservices with ease.
Complete documentation is available at https://traefik.io

Usage: traefik [flags] <command> [<arguments>]

Use "traefik <command> --help" for help on any command.

Commands:
        bug                                                Report an issue on Traefik bugtracker
        healthcheck                                        Calls traefik /ping to check health (web provider must be enabled)
        storeconfig                                        Store the static traefik configuration into a Key-value stores. Traefik will not start.
        version                                            Print version

Flag's usage: traefik [--flag=flag_argument] [-f[flag_argument]] ...     set flag_argument to flag(s)
          or: traefik [--flag[=true|false| ]] [-f[true|false| ]] ...     set true/false to boolean flag(s)

Flags:
...

[nmengin]

LGTM

[ldez]

LGTM