Handle file opening error

Don't segfault if the brightness file cannot be opened.

Support building with for different vendors

The vendor backlight directory was previously hardcoded as /sys/class/backlight/intel_backlight. This leads to the project not building on systems with a different vendor backlight directory.

This release tries to address the problem by using a heuristic to find the vendor backlight directory.

Incorporate community feedback

• Use setuid root over adding a sudoers exception
• Use install to install the program after compilation, set the owner, group, mode, and suid bit, and don't rely on the folders already existing
• Include a license

First public release

First public release.