Applied experimental patch from @welash. Now we can do at least Zork 1.

Applied experimental patch from @welash.  Now we can do at least Zork 1.

Two things were done here.

1.  Commented out assembly macros of SET_WORD and LOW_WORD, temporarily
using the Unix versions.  I'm examinging some ways of cleanly allowing
for a structure member to be passed into an assembly macro.  Turbo C /
Turbno Assembler has a problem doing a.flags when there also exists
b.flags.  It can't tell the difference and so will throw an error about
an ambiguous member name.  This happened with z_header.flags.

2.  Turbo C doesn't like seeing things like `z_header_t z_header;` in
multiple source files.  These were marked `extern` in all but one of
them.