Hopefully this will be the last commit.

Changes are as follows:

1. avahi-daemon (both .service and .socket) are unmasked when ujust toggle-cups is ran.
2. avahi-daemon.service and avahi-daemon.socket are disabled. As ipp-usb.service will automatically start avahi-daemon when a printer/scanner is connected. avahi-daemon does not have to be enabled to make plug-n-play printing/scanning work.
3. Default systemd unit files of avahi-daemon are overridden. Two systemd.unit options are toggled, StopWhenUnneeded and RefuseManualStart. The first option stops avahi-daemon when it is not in use, meaning once the printer/scanner is disconnected, avahi-daemon will be stopped. This would cause no breakage since it only exits when no system service Wants/Requires it. The second option disallows manually starting avahi-daemon to avoid confusion. Since avahi-daemon immediately exits when it's started manually, without any system services Want-ing/Require-ing it. Disallowing manual start of avahi-daemon would have no impact on ordinary users.
4. The overriding systemd unit files are removed when CUPS is disabled, and avahi-daemon (both .service and .socket) are re-masked.

Impacts:

1. When CUPS is enabled, avahi-daemon is started on-demand. Meaning that it will be started automatically when needed, and stopped when unused. These all happen automatically, without the need of users' intervention.