Ensure version slot of preloaded systems is correct after an upgrade

Fixes #94 (closed)

While doing this I discovered that:

• This bug also expresses itself if a newer asdf.lisp is loaded directly (instead of going through upgrade-asdf).
• The version number on the ASDF system itself is also broken if you load a newer asdf.lisp on top of an existing ASDF.

That meant approach 3 was the only feasible solution, as approaches 1 and 2 would only work if the .asd files are available.

Draft until I also fix upgrade-asdf. Its docstring is wrong: it currently returns T unconditionally, which causes the first operate call in any given ASDF session to bail out early and restart, as if operate may have been redefined.