user_level & capabilities tables prefixing need easy configuration

[dbasulto]

If you want to share users between 2 WP installs you can change the table prefixes for users and users_meta in wp-config.php. But you also need to make sure it points to the right user_level and capabilities tables, that are a bit harder to change, needing to edit capabilities.php

Perhaps a $users_prefix in wp-config (if set) will make WP look for user tables in another install tables. If it's not set, then it uses the default prefix, looking for users on its own tables.

[Denis-de-Bernardy]

dp of #8663