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/ask-password-api.c

@@ -730,8 +730,7 @@ int ask_password_agent(
         if (keyname)
                 (void) add_to_keyring_and_log(keyname, flags, l);
 
-        *ret = l;
-        l = NULL;
+        *ret = TAKE_PTR(l);
         r = 0;
 
 finish: