Add PageData C type

This adds the C type PageData and makes the existing type Page a
pointer to it.  This follows the usual PostgreSQL C type naming scheme
of Foo/FooData pairs.  (Prior to commit ddbba3a, PageData existed
as an unrelated type.)  The type definitions are compatible, so this
doesn't change anything except some of the naming.

Discussion: https://www.postgresql.org/message-id/flat/692ee0da-49da-4d32-8dca-da224cc2800e@eisentraut.org

Author: petere

src/include/storage/bufpage.h

@@ -78,7 +78,8 @@ extern PGDLLIMPORT bool ignore_checksum_failure;
  * fields.
  */
 
-typedef Pointer Page;
+typedef char PageData;
+typedef PageData *Page;
 
 
 /*

src/tools/pgindent/typedefs.list

@@ -1986,6 +1986,7 @@ PX_Combo
 PX_HMAC
 PX_MD
 Page
+PageData
 PageGistNSN
 PageHeader
 PageHeaderData