Converts a binary image file produced by makebin and adds the prefix
necessary to allow it to be used by z80-mon.
A typical workflow for assembling (non-relocatable) code and running in
z80-mon is as follows.
zmac -o foobar.hex foobar.asm
makebin foobar.hex foobar.bin
z80-fix <foobar.bin >foobar.z80
z80-mon foobar.z80