Add a function to get the authenticated user ID.

Previously, this was not exposed outside of miscinit.c.  It is needed
for the pending pg_background patch, and will also be needed for
parallelism.  Without it, there's no way for a background worker to
re-create the exact authentication environment that was present in the
process that started it, which could lead to security exposures.

src/backend/utils/init/miscinit.c

@@ -222,6 +222,16 @@ SetSessionUserId(Oid userid, bool is_superuser)
 	CurrentUserId = userid;
 }
 
+/*
+ * GetAuthenticatedUserId - get the authenticated user ID
+ */
+Oid
+GetAuthenticatedUserId(void)
+{
+	AssertState(OidIsValid(AuthenticatedUserId));
+	return AuthenticatedUserId;
+}
+
 
 /*
  * GetUserIdAndSecContext/SetUserIdAndSecContext - get/set the current user ID