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/basic/parse-util.c

@@ -324,8 +324,7 @@ int parse_syscall_and_errno(const char *in, char **name, int *error) {
                 return -EINVAL;
 
         *error = e;
-        *name = n;
-        n = NULL;
+        *name = TAKE_PTR(n);
 
         return 0;
 }