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/nspawn/nspawn.c

@@ -1175,8 +1175,7 @@ static int parse_argv(int argc, char *argv[]) {
                  * accept this here, and silently make "--ephemeral --template=" equivalent to "--ephemeral
                  * --directory=". */
 
-                arg_directory = arg_template;
-                arg_template = NULL;
+                arg_directory = TAKE_PTR(arg_template);
         }
 
         if (arg_template && !(arg_directory || arg_machine)) {