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/shared/fdset.c

@@ -25,6 +25,7 @@
 
 #include "sd-daemon.h"
 
+#include "alloc-util.h"
 #include "dirent-util.h"
 #include "fd-util.h"
 #include "fdset.h"
@@ -168,8 +169,7 @@ int fdset_new_fill(FDSet **_s) {
         }
 
         r = 0;
-        *_s = s;
-        s = NULL;
+        *_s = TAKE_PTR(s);
 
 finish:
         /* We won't close the fds here! */