memdisk: Use MEMDISK header to access "safe hook"

memdisk: Use MEMDISK header to access "safe hook"

This commit moves the MEMDISK header structure into
the common structures header file.

It also adds the "safe hook" structure to the MEMDISK
header structure, since that matches what's in the hook
binaries.  Thus, we access the "safe hook" via the
header pointer, instead of separately.

Tested against a DOS floppy image including an
experimental MDISKCHK.COM and also gainst WinVBlock.

Signed-off-by: Shao Miller <shao.miller@yrdsb.edu.on.ca>