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/busctl/busctl-introspect.c

@@ -275,10 +275,9 @@ static int parse_xml_node(Context *context, const char *prefix, unsigned n_depth
 
                                 free(node_path);
 
-                                if (name[0] == '/') {
-                                        node_path = name;
-                                        name = NULL;
-                                } else {
+                                if (name[0] == '/')
+                                        node_path = TAKE_PTR(name);
+                                else {
 
                                         if (endswith(prefix, "/"))
                                                 node_path = strappend(prefix, name);