copy: change the various copy_xyz() calls to take a unified flags parameter

This adds a unified "copy_flags" parameter to all copy_xyz() function
calls, replacing the various boolean flags so far used. This should make
many invocations more readable as it is clear what behaviour is
precisely requested. This also prepares ground for adding support for
more modes later on.

src/tmpfiles/tmpfiles.c

@@ -1170,7 +1170,7 @@ static int create_item(Item *i) {
                         return log_error_errno(r, "Failed to substitute specifiers in copy source %s: %m", i->argument);
 
                 log_debug("Copying tree \"%s\" to \"%s\".", resolved, i->path);
-                r = copy_tree(resolved, i->path, false);
+                r = copy_tree(resolved, i->path, COPY_REFLINK);
 
                 if (r == -EROFS && stat(i->path, &st) == 0)
                         r = -EEXIST;