memdisk: Move common structures into headers

memdisk: Move common structures into headers

Since MDISKCHK.COM (and possibly COM32 modules) could make use of
structures found in MEMDISK's setup.c, we move these into a new
header file mstructs.h.

Also, since the OpenWatcom compiler uses a structure packing prefix
and GCC uses a structure packing postfix, we accomodate this with
preprocessor definitions around those structures needing to be
packed.

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