settings: avoid calling superuser() in contexts where it's not defined,
don't leak the transient copies of GetConfigOption output, and avoid the
whole exercise in postmaster child processes.

I found that actually no current caller of GetConfigOption has any use for
its internal check of GUC_SUPERUSER_ONLY.  But rather than just remove
that entirely, it seemed better to add a parameter indicating whether to
enforce the check.

Per report from Simon and subsequent testing.