Fix some bugs in the floating-point adder.

To test the floating point code, I hacked together a simple translator
from the official Verilog implementation to C, and compared the output
of that with the emulator's implementation.
Move SCREEN_WIDTH/HEIGHT to risc.h file

Since it's a property of the RISC machine.
Add gitignore file and clean up a bit
Support filesystem-only images.

Patch from Paul Reed.
Warn about old disk images
Add PCLink shell scripts
Make it easy to override default sdl2-config
Fix break scancode of some "grey" keys

Fixes #1, reported by ilovb
The S_IRGRP, S_IWGRP, S_IROTH, and S_IWOTH macros are not defined on MinGW
Merge pull request #3 from ilovb/master

build fix for MinGW