Add code to apply some simple sanity checks to the header fields of a

page when it's read in, per pghackers discussion around 17-Feb.  Add a
GUC variable zero_damaged_pages that causes the response to be a WARNING
followed by zeroing the page, rather than the normal ERROR; this is per
Hiroshi's suggestion that there needs to be a way to get at the data
in the rest of the table.

src/backend/utils/misc/postgresql.conf.sample

@@ -213,6 +213,6 @@
 #sql_inheritance = true
 #transform_null_equals = false
 #statement_timeout = 0		# 0 is disabled, in milliseconds
+#zero_damaged_pages = false     # set this true only for disaster recovery
 #db_user_namespace = false
 #preload_libraries = ''
-