"persist" makes pppd not exit, but just redial, thus it doesn't deallocate
the unit number. 

] Train travel features AC outlets with no take-off restrictions|  firewalls  [
]   Michael Richardson, Sandelman Software Works, Ottawa, ON    |net architect[
] mcr@sandelman.ottawa.on.ca http://www.sandelman.ottawa.on.ca/ |device driver[
] panic("Just another NetBSD/notebook using, kernel hacking, security guy");  [