Remove zero_damaged_pages from postgresql.conf.sample; the only way to

Remove zero_damaged_pages from postgresql.conf.sample; the only way to
find out about it is to read the documentation that tells you how
dangerous it is.  Add default_transaction_read_only to documentation;
seems to have been overlooked in patch that added read-only transactions.
Clean up check_guc comparison script, which has been suffering bit rot.