Problem: RFC 27 allows the scheduler to send a partial-ok flag
in the hello request, and then receive partially allocated jobs
in hello responses.

If the hello request includes this flag, pass it on to housekeeping.
For each partially released housekeeping job, include the 'free'
idset in the response per RFC 27.

Problem: libschedutil provides no way for the scheduler to
indicate that the partial-ok flag should be set in the hello
request.

Add the SCHEDUTIL_HELLO_PARTIAL_OK flag which is passed to
schedutil_create().

Problem: when processing hello responses, all schedulers now need
to process R - free for partial releases.

As a convenience, change the libschedutil hello callback to subtract
the free idset from the R it fetched from the KVS.

Note that the scheduling key, if present, remains the full object
which is opaque to flux-core.

Problem: sched-simple does not support partial hello responses.

Set the SCHEDUTIL_HELLO_PARTIAL_OK flag.
Add a 'test-hello-nopartial' module option to get the old behavior.

Set test-hello-nopartial in the current test of partial housekeeping
release.

Problem: there is no coverage of reloading the scheduler with
partially released jobs in housekeeping.

Add a test.

Problem: when the hello protocol cannot process a job, it logs
the name of the wrong rlist function.

Make the log message a little more high level.