There are currently 5 types of keyboard mapping supported: D1, D13270,
D13270S, D2 and D23270. The D2 mappings are the CERN default for a
VT200-style keyboard, i.e. one which has the middle keypad with the keys
Find, Insert Here, Remove etc.

The D1 types of mapping are for terminals of type VT100 and Pericom PA/PG/P7.
These terminals have both a delete key and a backspace key (unlike VT200,
but like the Falcos), but do not have the special keys mentioned above.
On D1 mappings backspace is just like cursor left, whereas delete is treated
as a backwards tab (to the previous field).

Normally, the obvious keys such as cursor movement, tab, Return and so on
have the expected effect. The PF keys are frequently on the keypad (right
or left) or on the top line of function keys. In case all else fails then
the various PF and PA keys can be got by ESC (the Escape key or Ctrl [ ) :-
ESC 1,2,...,0 : PF1,PF2,...,PF10      ESC !,@,...,) : PF11,PF12,...,PF20
ESC - : PF21       ESC = : PF22       ESC _ : PF23       ESC + : PF24
ESC a1,a2,a3 : PA1,PA2,PA3
and normally Ctrl g = reset, Ctrl l = clear screen, Ctrl t = redraw screen
