Add -e option: Interrupt via Ctrl+C, Proper Makefile, and Cleanup sl.h

Add -e option to allow interrupt via Ctrl+C. So users can, `alias sl='sl -e'` if they don't want to wait each time for sl to finish running.


* Makefile now includes: install, uninstall, and clean. Therefore, users can install sl by running:
```
make
make install
```
* Add header guards to `sl.h`, and move function prototypes to `sl.h`.
* `README.ja.md` and `sl.1.ja` still need to be updated (I don't know Japanese).