app-admin/cancd: modernize the service script and ebuild.

app-admin/cancd: modernize the service script and ebuild.

This commit creates a new revision (-r4) that replaces the earlier
unstable -r3. The ebuild itself includes an update to EAPI=6 and the
associated simplifications, as well as the addition of a new user
named "cancd" that is used by the init script.

The service script was completely rewritten and simplified, and now
uses more modern features of OpenRC. The original motivation for
that was bug 603208.

This is a non-maintainer commit, but robbat2 (the maintainer on file)
recently made a post to gentoo-dev, "Packages up for grabs: robbat2
edition," stating that this package was up for grabs.

Closes: https://bugs.gentoo.org/603208
Package-Manager: Portage-2.3.13, Repoman-2.3.3