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/core/dynamic-user.c

@@ -770,8 +770,7 @@ int dynamic_user_lookup_uid(Manager *m, uid_t uid, char **ret) {
         if (check_uid != uid) /* lock file doesn't match our own idea */
                 return -ESRCH;
 
-        *ret = user;
-        user = NULL;
+        *ret = TAKE_PTR(user);
 
         return 0;
 }