Shorten logic in `Guard::login_message()`
New generic `guard_is_site_protected()` for both single and multisite installs
Update `guard_is_site_protected()` and implement its usage
Merge `guard_is_user_allowed_for_blog()` into `guard_is_user_allowed()` with a new $site_id param
Update phpDoc and functional logic for functions in functions.php
Introduce `guard_is_network_protected()` as a wrapper for `get_site_option()`
Introduce wrapper for network redirect option and put it to use