[Simh] Adding Unix v0 support to PDP18b

[Bob Supnik]

1. Create an RB09 variant at half size and allow it to be configured in 
the PDP7. No need for dynamic size support; Unix v0 supports one size 
(8000 sectors), and PDP9 software supports one size (16000 sectors).

2. Create a card reader for the PDP7. With minor mods, the same device 
could support the PDP-4 and PDP-1, but there's no point. See my paper on 
18b card readers for the sordid details.

3. Add a -u examine/deposit switch.

- On the PDP7/9, with 18b devices, it would mean "Unix format" ASCII - 
one character in each 9b byte.
- On the PPD15, with 16b or 18b devices, it would mean "Unichannel 
(PDP11)" ASCII - two 8b characters in 16b, little endian.

I think that should do it.

/Bob