improve UX for "--help" to show help pages

Version: 4.20.0-2 on debian 10

In order to improve UX for the "ip" command line utility provided by this package, I propose there should be more consistent use of the "--help" command within the application.

Right now, and as many users expect it to, "ip --help" will print the full usage page. This is in line with many other tools.

But when using the "--help" flag on any object such as "ip link --help", the following message is printed:

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~# ip link --help
Command "--help" is unknown, try "ip link help".
~#
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In my opinion, this message creates unnecessary friction for the user.
It should immediately show the same output as "ip link help" would.

Therefore, I propose to internally link the (currently unknown) command "--help" to the handler for the proper "help" command in order to increase usability.

I'm happy to provide a PR if the concensus is that this is a good addition to iproute2.