section config: update code style

section config: update code style

Replace `foreach` with `for` and use postfix deref instead of block
(circumfix) dereference (`$foo->%*` instead of `%$foo`).

Furthermore, make `format_config_line` a private sub instead of
unnecessarily declaring it as an anonymous subroutine, which avoids
the `&$sub_ref(...)` syntax altogether.

Signed-off-by: Max Carrara <m.carrara@proxmox.com>