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/nspawn/nspawn-mount.c

@@ -308,8 +308,7 @@ int overlay_mount_parse(CustomMount **l, unsigned *n, const char *s, bool read_o
                     !source_path_is_valid(lower[1]))
                         return -EINVAL;
 
-                upper = lower[1];
-                lower[1] = NULL;
+                upper = TAKE_PTR(lower[1]);
 
                 destination = strdup(upper[0] == '+' ? upper+1 : upper); /* take the destination without "+" prefix */
                 if (!destination)
@@ -321,8 +320,7 @@ int overlay_mount_parse(CustomMount **l, unsigned *n, const char *s, bool read_o
                  * the "upper", and all before that the "lower" directories. */
 
                 destination = lower[k - 1];
-                upper = lower[k - 2];
-                lower[k - 2] = NULL;
+                upper = TAKE_PTR(lower[k - 2]);
 
                 STRV_FOREACH(i, lower)
                         if (!source_path_is_valid(*i))

src/nspawn/nspawn.c

@@ -1175,8 +1175,7 @@ static int parse_argv(int argc, char *argv[]) {
                  * accept this here, and silently make "--ephemeral --template=" equivalent to "--ephemeral
                  * --directory=". */
 
-                arg_directory = arg_template;
-                arg_template = NULL;
+                arg_directory = TAKE_PTR(arg_template);
         }
 
         if (arg_template && !(arg_directory || arg_machine)) {