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/process-util.c

@@ -623,8 +623,7 @@ int get_process_environ(pid_t pid, char **env) {
         } else
                 outcome[sz] = '\0';
 
-        *env = outcome;
-        outcome = NULL;
+        *env = TAKE_PTR(outcome);
 
         return 0;
 }