vax/ddb(4): clean up machine dependent code and improve usability

Hi,

I've recently fixed a few bugs on VAX, and while I did that I noticed
that ddb(4) on VAX is a pain to work with. Here's my attempt fix this:

https://github.com/NetBSD/src/compare/trunk...hrosenfeld:NetBSD-src:vax-ddb

Since this is a rather large change, I'd like to give anyone interested
a chance to do a proper code review before I commit this. I've been
working with and on this for the past two weeks and tested this
extensively.


Hans


-- 
%SYSTEM-F-ANARCHISM, The operating system has been overthrown