- Use NAME_MAX and PATH_MAX instead of hardcoded values
- Allow paths to be longer than 256 chars
- Check pointers everywhere
- Use strncpy/snprintf instead of strcpy/sprintf
- Validate controllers' name (-s flag + a very long name -> bad things happening)
- Get rid of globals for dir iteration

- Rewrite of all clamp operations
- Get rid of unreachable code (the 'else' in the last chunk)

Now we can save, restore, get [max] brightness etc. for both screen
and keyboard controllers

This function was:
- Reading values from the controller
- Checking write permission even when we just want reading values
- Checking the mincap file instead of the actual controller

The last one is also true for other functions using
'light_getBrightness'.
This commit fixes that.