Convert DOS interface to use z_header structure.

Convert DOS interface to use z_header structure.

There is currently a problem with compiling with Turbo C 3.00

src\common\fastmem.c
Error src\common\fastmem.c 160: Ambiguous member name 'flags' in function
restart_header
Error src\common\fastmem.c 435: Ambiguous member name 'flags' in function
init_memory
*** 2 errors in Compile ***

        Available memory 4059444

** error 1 ** deleting src\common\fastmem.o
C:\FROTZ>