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/sysv-generator/sysv-generator.c

@@ -741,8 +741,7 @@ static int acquire_search_path(const char *def, const char *envvar, char ***ret)
         if (!path_strv_resolve_uniq(l, NULL))
                 return log_oom();
 
-        *ret = l;
-        l = NULL;
+        *ret = TAKE_PTR(l);
 
         return 0;
 }