Internals manual update

[Stan Shebs]

So as I've been refreshing my memory about GDB code, I've also been 
going through the internals manual and making it be more consistent with 
the current state of things. Would people be content with me posting a 
mega-patch(es) for overall progress, or would they prefer individual 
patches for each specific change?

Also, one specific change I made was to whack down the host stuff to 
basically one line that says "we used to have host config files", 
instead of a full section with a warning at the top that the material is 
of historical interest only. My thought is that if someone really wanted 
to research a long-gone macro more thoroughly than just review of 
ChangeLog entries, they would want to get old releases anyway, which 
would have both internals manual and sources using that macro. In 
general it seems to me that the internals manual should correspond to 
current sources closely, only have the occasional reference to ancient 
history.

Stan