Working, slowly, on a BASIC.SYSTEM clone of sorts

[Lyrical Nanoha <LyricalNanoha@dosius.net>]

I've thought of trying to cruft together a BASIC.SYSTEM replacement, but 
I'm not very familiar with any of what I'm doing.  Oh well.  Well, here's 
what I'm trying to do.

1. Interface more like DOS 3.3, with chr$(4) recognized even in direct 
mode and ASM programs, and MAXFILES working like DOS 3.3.  Attempt to 
limit code size to comparable with DOS 3.3 (since most of the OS is above 
$D000 anyway).  This should make DOS 3.3 programs more compatible with it.

2. Commands BYE, CREATE, PREFIX like BASIC.SYSTEM (replacing INIT, INT and 
MON).  Also, line and filetype semantics implemented in BASIC.SYSTEM may 
also need to be implemented here.  Also, add - command.  CATALOG is CAT in 
BASIC.SYSTEM.

I figure this might make some DOS 3.3 stuff easier to run on ProDOS as 
PDOS.SYSTEM did RDOS.

-uso.