Refactor wal.c to use the VFS.  This check-in compiles and links and works
ok as long as you leave WAL turned off, but WAL does not work.

FossilOrigin-Name: 62db5fa3b61be885b2d94e9b9ce3877b2c588350
diff --git a/src/sqliteInt.h b/src/sqliteInt.h
index 7fbe053..54effce 100644
--- a/src/sqliteInt.h
+++ b/src/sqliteInt.h
@@ -823,8 +823,8 @@
   void (*xRollbackCallback)(void*); /* Invoked at every commit. */
   void *pUpdateArg;
   void (*xUpdateCallback)(void*,int, const char*,const char*,sqlite_int64);
-  int (*xLogCallback)(void *, sqlite3 *, const char *, int);
-  void *pLogArg;
+  int (*xWalCallback)(void *, sqlite3 *, const char *, int);
+  void *pWalArg;
   void(*xCollNeeded)(void*,sqlite3*,int eTextRep,const char*);
   void(*xCollNeeded16)(void*,sqlite3*,int eTextRep,const void*);
   void *pCollNeededArg;