This adds a `wantedBy` clause to the user systemd service for
yubikey-agent, to ensure an enabled agent is started on boot. This
brings the behavior inline with existing documentation.
If the user has selected a graphical pinentry program, then we need to
wait for the graphical environment to exist before starting the
yubikey-agent. I've found that if we start the agent earlier it will
fail when we perform an ssh command later.
This adds yubikey-agent as a package and a nixos module.
On macOS, we use `wrapProgram` to set pinentry_mac as default in PATH;
on Linux we rely on the user to set their preferred pinentry in PATH.
In particular, we use a systemd override to prefix PATH to select a
chosen pinentry program if specified.
On Linux, we need libnotify to provide the notify-send utility for
desktop notifications (such as "Waiting for Yubikey touch...").
This might work on other flavors of unix, but I haven't tested.
We reuse the programs.gnupg.agent.pinentryFlavor option for
yubikey-agent, but in doing so I hit a problem: pinentryFlavour's
default value is specified in a mkDefault, but only conditionally. We
ought to be able to pick up the pinentryFlavour whether or not gpg-agent
is running. As a result, this commit moves the default value to the
definition of programs.gnupg.agent.enable.
