htmltidy: Avoid breaking the sidebar when websetup is running.

htmltidy: Avoid breaking the sidebar when websetup is running.

Problem was this: websetup loads all plugins, but does not checkconfig
them. So, htmltidy's recently added configurable command setting was unset;
this resulted in its sanitize hook failing; the sanitize hook is called
when a sidebar was enabled, and this caused the sidebar to not display.

I put in a fix, but the underlying problem is that websetup loads all
plugins but leaves them in an unconfigured and possibly broken state while
trying to display its forms.

Probably the long-term fix is to have it cache the original hook states from
before loading the plugins, and restore it after getting their configuration.
Or, even to get the configuration using a subprocess, as plugins may do things
outside the hook system.