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/import/importd.c

@@ -1083,8 +1083,7 @@ static int transfer_node_enumerator(sd_bus *bus, const char *path, void *userdat
                 k++;
         }
 
-        *nodes = l;
-        l = NULL;
+        *nodes = TAKE_PTR(l);
 
         return 1;
 }

src/import/pull-common.c

@@ -124,8 +124,7 @@ int pull_find_old_etags(
                         return r;
         }
 
-        *etags = l;
-        l = NULL;
+        *etags = TAKE_PTR(l);
 
         return 0;
 }