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/proc-cmdline.c

@@ -204,10 +204,8 @@ int proc_cmdline_get_key(const char *key, unsigned flags, char **value) {
                 }
         }
 
-        if (value) {
-                *value = ret;
-                ret = NULL;
-        }
+        if (value)
+                *value = TAKE_PTR(ret);
 
         return found;
 }