copy: Add support for creating subvolumes to copy_tree_at()

The subvolumes set is a set of source inodes similar to how the
denylist hashmap contains source inodes as keys. It indicates
directories in the source tree that should become subvolumes in
the target tree.

src/tmpfiles/tmpfiles.c

@@ -1866,7 +1866,7 @@ static int copy_files(Context *c, Item *i) {
                          i->uid_set ? i->uid : UID_INVALID,
                          i->gid_set ? i->gid : GID_INVALID,
                          COPY_REFLINK | ((i->append_or_force) ? COPY_MERGE : COPY_MERGE_EMPTY) | COPY_MAC_CREATE | COPY_HARDLINKS,
-                         NULL);
+                         NULL, NULL);
 
         fd = openat(dfd, bn, O_NOFOLLOW|O_CLOEXEC|O_PATH);
         if (fd < 0) {