Check hook callback is not boolean

Is your feature request related to a problem?

A client wanting to return false to a filter, did:

	add_filter( 'liveblog_entry_enable_embeds', false );

...instead of:

	add_filter( 'liveblog_entry_enable_embeds', '__return_false' );

Describe the solution you'd like

When a boolean false or true is given as the second arg of add_filter() or add_action(), make it an Error with a suggestion of what to use instead.

This could be extended to look for other __return_() functions, like null, 0, array(), etc.

Perhaps an error with a more vague message if it's anything non-callable?

Search the repository for the checks and tests covering the add_filter() and add_action() entry points, then trace how invalid callback arguments are reported. Define done as producing an Error for boolean callbacks with a suggestion to use the corresponding _return* function; the broader non-callable cases are optional in the request.