hosts 1.0.0

The first release that can be installed with apt, and the point from which
the interface is a promise.

Nothing in the program itself changed from 0.6.0. What changed is that there
is now a way to install it that does not begin with cloning a repository, and
a commitment about what will not break.

Added

• A Debian package, hosts-cli, holding the command, the man page and the
  completions for bash and zsh. make deb builds it from the same artifacts
  make install uses.
• An APT repository published on GitHub Pages, signed, and rebuilt from the
  packages attached to every GitHub Release so that it holds no state of its
  own and cannot drift from what was published.
• A release workflow that runs on a tag, refuses to continue when the tag and
  the VERSION file disagree, installs the package it just built and uses it
  before publishing anything, and takes the release notes from this file.

Changed

• GitHub Pages is deployed by a workflow rather than from the docs folder of
  the branch, because an APT repository contains a binary and committing one
  would put it in the history for good.
• The following are now covered by semantic versioning, and breaking any of
  them costs a major version: the exit codes 0 to 8; the tab separated output
  of ls, search, check, backup ls, profile ls and flush, the number
  of fields included; the JSON schema; the rule identifiers of check; the
  environment variables and the layout of the backup and profile stores,
  sidecar format included.