Added volume control and initial screen brightness control code to Linux. Updated dependencies and added template file for future operating system support.
Added audio volume control to Linux. Added initial code for changing screen brightness on Linux and updated dependnecy list. Added template file for future operating system support.
The following features have been added to the Linux port:
Added battery level detection and AC/battery detection.
Added detached audio mixer. Confirmed screen brightness adjustment works.

Also cleaned up DEPENDENIES file, separating FreeBSD and Linux requirements.
Both sections of the file now include full package/port names of
dependencies on both operating systems to make it easy to get all
build requirements, runtime requirements and optional components.
Merge pull request #3 from slicer69/linux

Audio volume and screen brightness added for Linux