Facilitate usage of nerdctl as a library to build other clients

### What is the problem you're trying to solve

I do maintain a fork for my own use, that contains radical changes that do not fit here.

As such, I want "my" nerdctl binary to be named differently, and also use different config, storage locations and labels names to avoid conflicts.

Generally, people looking at building their own x-ctl should be able to leverage nerdctl codebase to do that.

The problem right now is that "nerdctl" is hardcoded in a bunch of places, making that task rather pedestrian and error prone.

### Describe the solution you'd like

Suggesting that "nerdctl" strings across the codebase are reviewed and at least turned into constants that would be easy to change - or better, maybe we overload global config with a "Name" property that would default to "nerdctl" of course.

Let me know what the team here thinks about this use-case, and possible solutions.

### Additional context

_No response_