Add context info to OAT_POST_CREATE security hook

Add context info to OAT_POST_CREATE security hook

... and have sepgsql use it to determine whether to check permissions
during certain operations.  Indexes that are being created as a result
of REINDEX, for instance, do not need to have their permissions checked;
they were already checked when the index was created.

Author: KaiGai Kohei, slightly revised by me