[syslinux] Enhanced MDISKCHK.COM and MEMDISK patches

[Miller, Shao]

Good day Bernd,

Thanks for the usage report for multiple MEMDISKs with an enhanced
MDISKCHK.COM!  This seems to be a prime example.

I am actually in the process of working on an IFMEMDSK.C32 module, which
could benefit from some of the changes submitted as part of the enhanced
MDISKCHK.COM.  I'm going to review these changes so I can help H. Peter
to feel at-ease about committing them. :)

If you would be so kind as to test any of this in your setup once ready,
I'd really like that.

- Shao Miller