Changing filenames

Changing two filenames:
- tests/gen.c    becomes tests/generator.c
- tests/makefile becomes tests/Makefile

That is obviously better to have the main file of the program
`generator` named `generator.c`.

The default naming of makefiles is Makefile with a capital
letter. Conforming to that rule is better.

Signed-off-by: José Bollo <jose.bollo@open.eurogiciel.org>
Renaming 'r' to 'alea'

Signed-off-by: José Bollo <jose.bollo@open.eurogiciel.org>
Correcting exit codes

The valid exit code are form 0 to 255. Then using -1 means that the
effective exit code will be 0377==255. It isn't accurate. The exit code
of 1 is merely very often used for failure status.

Signed-off-by: José Bollo <jose.bollo@open.eurogiciel.org>
Adding some comments

Signed-off-by: José Bollo <jose.bollo@open.eurogiciel.org>