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/journal/mmap-cache.c

@@ -220,8 +220,7 @@ static void context_detach_window(Context *c) {
         if (!c->window)
                 return;
 
-        w = c->window;
-        c->window = NULL;
+        w = TAKE_PTR(c->window);
         LIST_REMOVE(by_window, w->contexts, c);
 
         if (!w->contexts && !w->keep_always) {