Ensure default-only storage parameters for TOAST relations

to be initialized with proper values. Affected parameters are
fillfactor, analyze_threshold, and analyze_scale_factor.

Especially uninitialized fillfactor caused inefficient page usage
because we built a StdRdOptions struct in which fillfactor is zero
if any reloption is set for the toast table.

In addition, we disallow toast.autovacuum_analyze_threshold and
toast.autovacuum_analyze_scale_factor because we didn't actually
support them; they are always ignored.

Report by Rumko on pgsql-bugs on 12 May 2010.
Analysis by Tom Lane and Alvaro Herrera. Patch by me.

Backpatch to 8.4.