Support "extended attributes" for INT 15h, AX=E820h

Support "extended attributes" for INT 15h, AX=E820h

Some blithering idiot thought it was a good idea to introduce
"extended attributes" for INT 15h, AX=E820h, and in doing so, breaking
compatibility with ALL E820 users out there.  F*cking morons.
Implement handling of extended attributes in:

- e820 parsing in the core
- e820 parsing in libcom32
- e820 parsing *and proxying* in memdisk

The latter is the really painful one.

Signed-off-by: H. Peter Anvin <hpa@zytor.com>