Calc example works!

[Mark S. Miller <mark>]

To be found in ~mark/xanadu/xalpha/calc

It is a RPN desk calculator server which let's you multiply rational
numbers.  Yup that's all, it's a useless program by itself.  It
exercises enough of stubble to be a quite good intoductory example.
It also got me to encapsulate a lot of mechanism that hadn't been
general before for starting up both a client and a server and having
them talk to each other.  The client and server can run either over
stubble/sockets or linked together, and check out against the same
regression test.  *All* the calc specific source files are insensitive
to which way they're run!  README is a (too terse) explanation of the
source files in the directory.

I did break one thing in trying to abstract out the general mechanism
from the current code: command line argument interpretation.  Until
this is fixed, I can't change the xanadu code to use this new
interface to its old mechanisms.  In thinking about this, I find I'm
confused about command line argument interpretation.  Several of us
should discuss this.  Until this is resolved, the example isn't right.

Note that all this works now *only* in the context of
~mark/xanadu/xalpha which is derived from a pre-publication version of
alpha-4. 

Bobp, how about we get together after the Tuesday meeting and look all
this over?