I rely on hitting the `Next unread message' button to show me any unread
message in a mailbox, and right now it doesn't if the message is in a
thread that's collapsed.  The following patch replaces the search in the
index as a clist with a search in the index as a ctree, and expands thread
enough to make the node viewable.

The code for `Next flagged message' is identical, so I've made them both
wrappers for a `select_next_by_flag' function which does the real work.