searchthread: fix usage of user-supplied parameter
Instead of only passing an Inbox object, we'll pass the $ctx
reference as PublicInbox::SearchView::mset_thread did.
So although mset_thread was wrong, we now make it's usage
of SearchThread::thread correct and update other callers to
favor the new style of passing the entire $ctx (with ->{-inbox})
instead of just the Inbox object.
This makes the thread skeleton at the bottom of the search
page to show subjects of messages, but unfortunately links to
non-existent #anchors. The next commit will fix that.
While we're at it, favor "\&foo" over "*foo" since the former
makes the code reference (aka "function pointer) obvious so it
won't be confused for other things named "foo" in that
scope (e.g. $foo/@foo/%foo).