This is a type of physical link that can chat and talk
LCP & CCP. A bundle contains a list of these (only one
in the list for the moment).
The datalink is a type of descriptor, and dials, enters
LCP (& does CCP), kicks the bundle when its FSMs do
something interesting and does the hangup chat script
on the way down. It also handles redials and reconnects.
There are lots of loose ends, and probably lots of bugs,
but the data structures are getting there !
