[cmds] Enhance fsck-dos to work with large 32M hard drives

[ghaerr]

This now allows for 32MB and smaller hard drive DOS FAT images to be consistency checked.

When compiled with OWCC, fsck-dos.os2 uses huge pointers to work with the much larger FAT table found in ELKS 32MB FAT hard disk images. To check a 32MB hard drive, there must be > 256KB of contiguous main memory available, which may not be the case with multiple logins or shells running.

Hopefully this tool might help @tyama501 with the possible HD FAT corruption seen in #2398.

The source code for fsck-dos was somewhat easily enhanced to use __huge far pointers, which causes OWCC to generate additional code for indexing huge far pointers into > 64KB contiguous RAM allocated through fmemalloc. The standard malloc/free were then replaced with an enhanced version developed for the 8086-toolchain, where two far memory arenas are maintained, one for small allocations using _fmalloc, and the other using fmemalloc.

For the time being, this requires OWCC being installed on the host. To build the enhanced version, type:

# cd elkscmd/fsck_dos
# make fsck-dos.os2
# cp fsck-dos.os2 ../rootfs_template/root
# cd
# ./fsck-dos.os2 /dev/hda1

For those without OWCC, here's a pre-compiled binary:
fsck-dos.os2.zip