macro: introduce TAKE_PTR() macro

This macro will read a pointer of any type, return it, and set the
pointer to NULL. This is useful as an explicit concept of passing
ownership of a memory area between pointers.

This takes inspiration from Rust:

https://doc.rust-lang.org/std/option/enum.Option.html#method.take

and was suggested by Alan Jenkins (@sourcejedi).

It drops ~160 lines of code from our codebase, which makes me like it.
Also, I think it clarifies passing of ownership, and thus helps
readability a bit (at least for the initiated who know the new macro)

src/tmpfiles/tmpfiles.c

@@ -310,8 +310,7 @@ static int user_config_paths(char*** ret) {
         if (r < 0)
                 return r;
 
-        *ret = res;
-        res = NULL;
+        *ret = TAKE_PTR(res);
         return 0;
 }