errno values and other CPP constants

[Tom Lord]

I've found that it makes for a very nice interface to represent errno
values and other CPP constants as symbols (e.g. 'ENOENT).

A nice convention for errno is to reserve "%" as the first character
in the name of any function that might return an errno value.
The procedure "%%" is used to convert errno returns to exceptions:


        (%open "no-such-file") => ENOENT

        (%% %open "no-such-file") =>  ...causes exception...

-t