runlisp-base.conf: Etch SBCL's idea of where its `home' is.

runlisp-base.conf: Etch SBCL's idea of where its `home' is.

Older SBCL versions end up believing that their `home' directory is
wherever the image was loaded from, but this is wrong for our dumped
images.

The directory is set by the C runtime code and (rather unpleasantly)
passed through the `SBCL_HOME' environment variable to Lisp, which
retrieves it via the `sb-impl::sbcl-homedir-pathname' function.

On the affected older SBCL versions, when we're dumping an image,
replace `sb-impl::sbcl-homedir-pathname' with a version which just
parrots the pre-dumped value.

Mention in `README.org' that it doesn't describe the actual SBCL
configuration any more.  (Sadness.)