Support OS_VARIANT on Linux, support ChromeOS.

This adds a detection for Chrome OS and Chromium OS based on /etc/lsb-release,
which sets LOWER_VENDOR, like for other Linux distros. It also sets OS_VARIANT
to the value of LOWER_VENDOR, so we can have conditionals for ChromeOS. It is
missing some things that are silently assumed to be part of Linux base
systems, such as POSIX attr support, NIS and more.

ok jperkin@

(bsiegert)