notmuch-show.c: add an option for messages to be returned unthreaded

This adds a --unthreaded option to notmuch show to tell it to return
the matching messages in an unthreaded order (so just by date).

To make it easier for users, in particular for notmuch-tree.el, we
output each message with the same "nesting" as if it were an entire
thread in its own right.

amended by db: s/status= /status = /