os-release: add a new FANCY_NAME= field to /etc/os-release, similar to PRETTY_NAME...

It's sometimes useful include non-ascii unicode chars in an os name, and
give it some ansi coloring. Since we usualy don't want to show that,
introduce a new field for it, and show it at boot and in thostnamectl
only, with safe fallbacks if colors/emojis are not available.