[Linux] Read OS information from /etc/os-release

Most modern Linux distributions (including Ubuntu, Arch Linux, Fedora, SuSE, Gentoo etc.) use the /etc/os-release file for operating system information. This is in order to get rid of all the distro-specific release files. See the links below for details.

This patch will try to read the PRETTY_NAME variable from
/etc/os-release to identify the Linux distribution.
If /etc/os-release doesn't exist, the code will still fall back
to lsb_release or the distro-specific release files.

http://www.freedesktop.org/software/systemd/man/os-release.html
http://0pointer.de/blog/projects/os-release.html