Allow themes and plugins to disable options in the admin

[holizz]

I frequently set options in functions.php by hooking 'pre_option_*', thus whatever the user does in the admin interface they can't change it.

There should be a filter to make the option obviously disabled to the user (it should probably also set the disabled attribute on the form fields) and perhaps add a short message saying for instance "This option was changed by $plugin_name, visit $link to change it." or "This option has been set by the '$theme_name' theme. It cannot be changed."

This will require #16413 first.