Return correct exit code

Return correct exit code

When oggquiz(1) exits because you requested the usage help using
`-h` it's correct to return the exit code `0`. But if it shows
the usage help because you gave wrong flags `1` has to be
returned as exit code.

 * new function `usage`
 * use `printf` instead of `puts`
 * return exit code `0` if program terminates correctly, `1`
   otherwise