Sortix release/version/abi files
Upgrades to the next Sortix release, as well as automatic detection of existing installations, will need to know the release of an existing installation. We should probably have an /etc/os-release file, or at least an /etc/sortix-release (if the existence of /etc/os-release will make software think this is a linux, in a bad way). We might also want an /etc/abi or /lib/abi (or just put it in /etc/os-release) file that contains a major and minor kernel ABI version. Major not matching between source and target systems means it's unable to run binaries from the other system, or if the major matches but the minor is less. major increased on kernel ABI break, minor on kernel ABI addition (such as syscall).