Heads up: rewriting `struct kboard'

[Tom Tromey]

I am going to check in my `struct kboard' patch now.
This may break the build on some platforms.

It is very similar to the `struct buffer' patch.
It rewrites references to Lisp-visible fields of `struct kboard'
to use a new macro, KVAR.  The fields are renamed to prevent old-style
accesses.

As with the `struct buffer' patch, the fix for any given breakage is
simple: turn `kbd->field' into `KVAR (kbd, field)'.  Please do not
directly use KBOARD_INTERNAL_FIELD.

I wrote this patch using a hacked copy of the script I used for struct
buffer.  I can send it to anybody who is interested.

I think this is the last infrastructure patch that I need on the trunk.
Next I plan to make a new concurrency branch and start importing
relevant patches there.

Tom