EH improvements

Signed-off-by: Paul Brook <paul@nowt.org>
Tag system management

Signed-off-by: Paul Brook <paul@nowt.org>
Manage space state

Signed-off-by: Paul Brook <paul@nowt.org>
Don't auto-open when processing event backlog

Signed-off-by: Paul Brook <paul@nowt.org>
Add ARP scanning

Signed-off-by: Paul Brook <paul@nowt.org>
Use deadline presence detection

Signed-off-by: Paul Brook <paul@nowt.org>