Add table generator to 1984/mullender from author

This included adding the 'a.out.h' as not all systems have it and also
I wanted it as close to the original as possible so I got a copy
(https://raw.githubusercontent.com/dspinellis/unix-history-repo/Research-Release/usr/include/a.out.h) from
the fabulous Unix History repo
(https://github.com/dspinellis/unix-history-repo) which I highly
recommend anyone interested in the evolution of Unix to check out!

... and yes: the repo is under the account of Diomidis Spinellis who is
a previous IOCCC winner!

To generate the tool run `make gentab'. Note that keeping the short ints
short generates a lot of negative numbers but this might or might not be
a problem: the original mullender.c has a negative number for instance.

Updated .gitignore and README.md files.

1984/mullender/Makefile

@@ -131,6 +131,10 @@ ${PROG}: ${PROG}.c
 	@echo "than a Vax-11 or pdp-11."
 	${CC} ${CFLAGS} $< -o $@ ${LIBS}
 
+gentab: gentab.c
+	${CC} ${CFLAGS} $< -o $@ ${LIBS}
+
+
 # alternative executable
 #
 alt: data ${ALT_TARGET}
@@ -164,6 +168,7 @@ clean:
 clobber: clean
 	${RM} -f ${TARGET} ${ALT_TARGET}
 	${RM} -f core
+	${RM} -f gentab
 	@-if [ -e sandwich ]; then \
 	    ${RM} -f sandwich; \
 	    echo 'ate sandwich'; \