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/libudev/libudev-enumerate.c

@@ -69,7 +69,6 @@ struct udev_enumerate {
  **/
 _public_ struct udev_enumerate *udev_enumerate_new(struct udev *udev) {
         _cleanup_free_ struct udev_enumerate *udev_enumerate = NULL;
-        struct udev_enumerate *ret;
         int r;
 
         assert_return_errno(udev, NULL, EINVAL);
@@ -97,10 +96,7 @@ _public_ struct udev_enumerate *udev_enumerate_new(struct udev *udev) {
 
         udev_list_init(udev, &udev_enumerate->devices_list, false);
 
-        ret = udev_enumerate;
-        udev_enumerate = NULL;
-
-        return ret;
+        return TAKE_PTR(udev_enumerate);
 }
 
 /**