max_parallel_degree context level

Hi all,

As it currently stands, max_parallel_degree is set to a superuser
context, but we probably want to discuss whether we want to keep it
this way prior to releasing 9.6.  Might we want to reduce its level so
that users can adjust it accordingly?  They'd still be limited by
max_worker_processes, so they'd at least be constrained by that
setting.

Opinions?

Thom