Patch from Jean-Claude Beaudoin adding MKCL support.
Undo the "make unwriteable" in build/asdf.lisp target.

Breaks our release make targets.
Bump version to 3.1.0.96
Tweak the ASDF (ASDF/INTERFACE) package. Most notably export perform-with-restarts (long forgotten).
Also, stop uninterning long obsolete useless things that don't matter and are probably punted on.
Tweak comments.
Test image-op. Add missing implementation method (oops).
Try to fix bitrotted prebuilt-system, based on initial MKCL support.
prebuilt-system isn't currently tested, and will bitrot again if it is kept this way.
Some cleanups after MKCL support.
concatenate to a temporary file for more robustness.
Fix test-program, etc.
Bump version to 3.1.0.97
@node surgery on documentation.
Export test-load-systems from asdf-test.