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/shared/machine-image.c

@@ -80,7 +80,6 @@ Image *image_unref(Image *i) {
 
 static char **image_settings_path(Image *image) {
         _cleanup_strv_free_ char **l = NULL;
-        char **ret;
         const char *fn, *s;
         unsigned i = 0;
 
@@ -104,10 +103,7 @@ static char **image_settings_path(Image *image) {
         if (!l[i])
                 return NULL;
 
-        ret = l;
-        l = NULL;
-
-        return ret;
+        return TAKE_PTR(l);
 }
 
 static char *image_roothash_path(Image *image) {