Allow thread index view to sort oldest first

Allow thread index view to sort oldest first

This allows the thread index view to present oldest messages first
instead of always presenting newer messages at the top of the list.
This can be toggled using the 'o' key.

A config file option :inbox_newest_first controls whether this is
turned on by default when viewing the inbox

Refine searches inherit the order from the originating thread index.

Signed-off-by: Keith Packard <keithp@keithp.com>