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/main.c b/src/main.c
index 3e7c125..c723a4e 100644
--- a/src/main.c
+++ b/src/main.c
@@ -1197,9 +1197,9 @@
 ){
   void *pRet;
   sqlite3_mutex_enter(db->mutex);
-  pRet = db->pLogArg;
-  db->xLogCallback = xCallback;
-  db->pLogArg = pArg;
+  pRet = db->pWalArg;
+  db->xWalCallback = xCallback;
+  db->pWalArg = pArg;
   sqlite3_mutex_leave(db->mutex);
   return pRet;
 }