[Simh] Adding second copy of device to PDP-8

[David Gesswein]

   I was looking at adding a second TD controller to the PDP-8. It did
not appear that simh has an easy way of adding another copy of a device it
already knows about with a new device code etc.

My guess is I would need to modify the existing driver to duplicate
DEVICE td_dev and all structures referred to by it. It also looks like the
code directly references some of file global data so all direct references 
would need to be changed. Is this the general approach?