New output infrastructure - #1979

This PR introduces a lot of new stuff to lib part which is mostly related to generating the results.

The idea is, that a plugin has a main result object (of type mp_check) to which you add partial results, called subchecks (mp_subcheck).
This mp_subchecks can also contain mp_subchecks, so you have a tree of results.

Ideally we remove all of that specific logic/string handling and so on from the individual plugins, which would reduce complexity (in the plugins).
Also this would open the path to centrally structure and format output, which would be cool for me, since I have a hard time regarding the output of some plugins (cough check_disk).

The handling would be something like

mp_check main = mp_check_init();

subcheck1 = check_filesystem_A();
subcheck2 = check_filesystem_B();
subcheck3 = check_something_else();

mp_add_subcheck_to_check(main, subcheck1);
mp_add_subcheck_to_check(main, subcheck2);
mp_add_subcheck_to_check(main, subcheck3);

mp_exit(main);

No plugin was changed here, the functionality is implemented in
parallel to the existing stuff (mostly), so it can be included
and plugins can be adapted incrementaly following that (which I plan to do).

Problems

• This introduces features from C11/C17, so this will break backwards compatibility