[Linux] Read OS information from /etc/os-release
Most modern Linux distributions use the /etc/os-release
file for operating system information.
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
Merge pull request #2900 from mus65/os-release

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