libeventd-nd: Merge to nd plugin

Signed-off-by: Quentin Glidic <sardemff7+git@sardemff7.net>
nd/style: Do not use min_width < 0

Signed-off-by: Quentin Glidic <sardemff7+git@sardemff7.net>
nd/cairo: Add a fast path when no radius

Signed-off-by: Quentin Glidic <sardemff7+git@sardemff7.net>
nd/cairo: Optimise radius limit computation

Signed-off-by: Quentin Glidic <sardemff7+git@sardemff7.net>
eventd/control: Make "start" a no-op

Signed-off-by: Quentin Glidic <sardemff7+git@sardemff7.net>
eventdctl: Rework the start/auto-spawn logic

Signed-off-by: Quentin Glidic <sardemff7+git@sardemff7.net>