Management framework: additional logging tweaks

Ensure the framework's log stream exists prior to using it in zeek_init(), and
use a node-is-live message similar to those in agent and controller also in
launched nodes.

scripts/policy/frameworks/management/log.zeek

@@ -122,7 +122,8 @@ function error(message: string)
 			 $role=r2s[Management::role], $message=message]);
 	}
 
-event zeek_init()
+# Bump priority to ensure the log stream exists when other zeek_init handlers use it.
+event zeek_init() &priority=5
 	{
 	if ( ! Supervisor::is_supervised() )
 		return;