Starting to get MBASIC.com to work

[skx]

This pull-request allows MBASIC.COM to launch and run :)

Once complete it will close #49, and as part of the implementation changes introduced in this pull-request #50 will also be closed.

There are two main changes to get MBASIC running, and that is largely because it is not a "well-behaved" application:

• We setup fake entrypoints in the zero-page.
  • This ensures that the various RST XXX instructions will ultimately jump to our handlers.
  • This is necessary because mbasic is not well-behaved, and doesn't invoke CP/M functions via call 0x0005.
• We implemented the aux I/O routines.
  • The output routine attempts to convert vt52/adm3a to ANSI

This pull-request, as-is, works well enough to launch and operate mbasic however there are some spurious output characters that need to be eliminated..

[skx]

OK current state:

• tastybasic works.
• mbasic works
  • BUT it produces spurious output characters.

I can either fix the spurious output, and merge this. Or I can say "works - merge", and later fix it up.

The work to add the io/ package has resolve #50 though. I think.

[skx]

So previously I said:

BUT it produces spurious output characters.

All the code uses A_READ and A_WRITE for its functionality, however the spurious characters are coming from calls to C_WRITE - which should not be happening.

Since it is "illadvised" to mix AUX and CON functions I think I just disable the C_WRITE call after A_READ or A_WRITE have been used. Needs to be tested.

Unless something is wrong in my indirection, via OUT, I think this is a weird issue to see. Seems like the cause is easy to understand, but the reason? Bug? Unexpected thing I'm missing?