Do not attempt to accurately determine whether posters can read private messages in viewtopic

It takes 5 queries to determine whether a user can read private messages. viewtopic.php checks whether each poster can receive private messages. It does so regardless of anything else, whether private messages are enabled or not, whether the current user is logged or not, etc...

I propose to add a configuration setting in the "Load settings" category to determine whether viewtopic should accurately determine whether a user is permitted to read private messages or simply assume they can as long as private messaging is enabled. Regardless of this setting, the icon does not appear if private messaging is disabled, if the current user is not permitted to send private messages, etc...