Should get the OS name from os-release(5)

[LukeShu]

Currently the distro module gets the OS name by guessing Arch Linux if /etc/pacman.conf exists, and falling back to uname -o otherwise.

I propose that (per man 5 os-release) it check whichever of /etc/os-release and /usr/lib/os-release, is found first; and use PRETTY_NAME, fall back to NAME, fall back to ID, then finally fall back to uname -o.

Relatedly, having the default color be os-release:ANSI_COLOR wouldn't be a bad idea either.