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/strv.c

@@ -341,8 +341,7 @@ int strv_split_extract(char ***t, const char *s, const char *separators, Extract
                 if (!GREEDY_REALLOC(l, allocated, n + 2))
                         return -ENOMEM;
 
-                l[n++] = word;
-                word = NULL;
+                l[n++] = TAKE_PTR(word);
 
                 l[n] = NULL;
         }
@@ -353,8 +352,7 @@ int strv_split_extract(char ***t, const char *s, const char *separators, Extract
                         return -ENOMEM;
         }
 
-        *t = l;
-        l = NULL;
+        *t = TAKE_PTR(l);
 
         return (int) n;
 }