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/shared/install-printf.c

@@ -50,10 +50,8 @@ static int specifier_prefix_and_instance(char specifier, void *data, void *userd
                 if (!ans)
                         return -ENOMEM;
                 *ret = ans;
-        } else {
-                *ret = prefix;
-                prefix = NULL;
-        }
+        } else
+                *ret = TAKE_PTR(prefix);
 
         return 0;
 }