smcvpostcomment: always allow wikilinks, and do access control

wikilinks are harmless, so we might as well allow them.

Access control for this plugin is a bit odd, since we specifically
don't want to allow comments to be edited - so the check is whether the
user is allowed to edit a deliberately invalid page name,
page/commented/on[smcvpostcomment]. You can put smcvpostcomment(*)
or smcvpostcomment(some/subdir/*) in $config{anonok_pagespec}
or the opposite in $config{locked_pages} to allow "editing" (really
just posting) comments.