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/fs-util.c

@@ -458,10 +458,8 @@ int get_files_in_directory(const char *path, char ***list) {
                         n++;
         }
 
-        if (list) {
-                *list = l;
-                l = NULL; /* avoid freeing */
-        }
+        if (list)
+                *list = TAKE_PTR(l);
 
         return n;
 }
@@ -838,10 +836,9 @@ int chase_symlinks(const char *path, const char *original_root, unsigned flags,
                 }
 
                 /* If this is not a symlink, then let's just add the name we read to what we already verified. */
-                if (!done) {
-                        done = first;
-                        first = NULL;
-                } else {
+                if (!done)
+                        done = TAKE_PTR(first);
+                else {
                         /* If done is "/", as first also contains slash at the head, then remove this redundant slash. */
                         if (streq(done, "/"))
                                 *done = '\0';
@@ -863,10 +860,8 @@ int chase_symlinks(const char *path, const char *original_root, unsigned flags,
                         return -ENOMEM;
         }
 
-        if (ret) {
-                *ret = done;
-                done = NULL;
-        }
+        if (ret)
+                *ret = TAKE_PTR(done);
 
         if (flags & CHASE_OPEN) {
                 int q;