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/cap-list.c

@@ -103,8 +103,7 @@ int capability_set_to_string_alloc(uint64_t set, char **s) {
 
         str[n > 0 ? n - 1 : 0] = '\0'; /* truncate the last space, if it's there */
 
-        *s = str;
-        str = NULL;
+        *s = TAKE_PTR(str);
 
         return 0;
 }