nd: Keep the backend id around

Signed-off-by: Quentin Glidic <sardemff7+git@sardemff7.net>
nd: Precise which target we attached to

Signed-off-by: Quentin Glidic <sardemff7+git@sardemff7.net>
nd: Precise detached backend and target

Signed-off-by: Quentin Glidic <sardemff7+git@sardemff7.net>
nd: Add "status" command

Signed-off-by: Quentin Glidic <sardemff7+git@sardemff7.net>
nd: Add "backends" command

Signed-off-by: Quentin Glidic <sardemff7+git@sardemff7.net>
eventdctl: Split plugin commands to their own manpages

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