websetup: Allow enabling plugins listed in disable_plugins.

websetup: Allow enabling plugins listed in disable_plugins.

The bug here was that disabling a plugin included thru goodstuff, like
htmlscrubber, caused it to be added to disable_plugins, and those plugins
were never loaded, so could not be re-enabled. Fix by allowing them to be
force loaded when appropriate. (Also that allows disabled plugins to still
record their setup options when dumping a setup file.)