Somewhat more uniform style and a few additional bits.

Rather minor fixups for things changed in the meanwhile and not
yet (re)documented properly; and a change for memtest feature
to require syslinux feature (the code's been changed to fit
the updated description, actually, and the change is purely
formal as no syslinux alternative is being used/planned so far).

ToC:
- introduced doc/
- features.in/00example/
- READMEs and TODO for more
- code comments clarified

Some unrelated minor fixes bundled either.