dill: fall back to default session
This lets us support the "random userspace app sending dill belts".
Ultimately, we'll want to be able to specify a session identifier
alongside the belt, instead of strictly relying on the duct.
..^$(eye.all (~(del ju eye.all) duct.task hen))
::
=/ nus (ax hen)
+ =? nus &(?=(~ nus) ?=(^ hey.all))
+ ::TODO allow specifying target session in task
+ (ax u.hey.all)
?~ nus
:: :hen is an unrecognized duct
:: could be before %boot (or %boot failed)
