bugzilla emails now threaded

[Luis Villa <louie ximian com>]

Something I've been meaning to do for a while; now done. Most mail
readers should now see bugzilla emails as threaded. Mind you, not
threaded well- there is no way to do that without significant hackery
while this is a configuration parameter. [sort of ;) But threaded, which
should help some people deal with their bugzilla email a bit better. 

Hope this helps-
Luis