How do I specify a unit for ppp?

[Dan Langille]

>From man ppp I see this:

  The -unit flag tells ppp to only attempt to open /dev/tunN.

I want it to use tun0.  I can't see a way to specify a unit in 
/etc/rc.conf.

Clues?
-- 
Dan Langille : http://www.langille.org/