need a way to lookup local-uri for child instance on a node

[grondo]

The Flux PAM module needs to take the following steps to allow use of multi-user subinstances by an arbitrary guest:
(step 1 is the part that is implemented now)

1. Permit access if user has an active job in the root instance on the current node
2. else If there is an active job for user flux on the current node and this is rank 0 of that job: permit access
3. else open a handle to the child instance and read configuration, if access.allow-guest-user=false: deny access
4. else goto 1 using the child instance handle

We can currently only accomplish steps 1,2 here, since the PAM module has no way to discover the local-uri for a subinstance on an arbitrary rank of the job.

I wonder if there's some simple way subinstances with the same userid as their parent can publish their local-uri such that the PAM module can fetch it?

[grondo]

else If there is an active job for user flux on the current node and this is rank 0 of that job: permit access

Actually, probably should always check for access.allow-guest-user, but you get the idea (checking for rank 0 is what we can do now)