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/path-util.c

@@ -290,8 +290,7 @@ char **path_strv_resolve(char **l, const char *root) {
                 r = chase_symlinks(t, root, 0, &u);
                 if (r == -ENOENT) {
                         if (root) {
-                                u = orig;
-                                orig = NULL;
+                                u = TAKE_PTR(orig);
                                 free(t);
                         } else
                                 u = t;