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/specifier.c

@@ -117,8 +117,7 @@ int specifier_printf(const char *text, const Specifier table[], void *userdata,
                 *(t++) = '%';
 
         *t = 0;
-        *_ret = ret;
-        ret = NULL;
+        *_ret = TAKE_PTR(ret);
         return 0;
 }