Redundant functionality with NM

[lbeltrame]

A lot of the functionality of the plugin exposes details of wg-quick even though NM offers comparable facilties:

• Pre/post up/down are not needed: NM uses dispatcher scripts;
• DNS is useless on distributions that don't ship resolvconf, while NM's own DNS handling works
• MTU is not needed while NM can handle it

And so on. In the end a lot of functionality is duplicated, and in some cases (like DNS) does not even work on all systems.

The plugin should leverage first what NM offers.