Avoid potential name space collision for the global variables PC, SP and BC
when readline is dynamically loaded. For some unknown reason, ncurses
exports these internal variables which may be needed in the context of
how ncurses is used by readline implementations. In any case, these
variable names are common in simulators, so there is an undesired
interaction between simulator internal variables and the simplest
solution is to rename these simulator global variables where they exist.
Another alternative is to merely declare these variables static when
they happen to only be referenced in a single simulator source module.
a) Added sim_debug statements, formatted console messages.
b) Removed old debugging statements and redundant byte/word mask operations.
c) Added more device commands.
d) Added zx-200a and changed nulldev to return 0 vice 0xff.
e) Added commands and corrected RTYPE and RBYTE behavior so ISIS-II would boot.
f) Corrected device calls in SBC_reset, corrected get_mbyte and put_mbyte to
match control bits from ipc_cont.c.
