repo: Flip the fsync default to off for new checkout API

Since we're adding a new API, we have the opportunity to fix
the defaults.  We expect clients to do a `syncfs()` or equivalent
on their own now, since it's way more efficient.

Flip the checkout fsync default to off.

Closes: #425
Approved by: giuseppe

src/libostree/ostree-repo.h

@@ -748,8 +748,8 @@ typedef struct {
   OstreeRepoCheckoutMode mode;
   OstreeRepoCheckoutOverwriteMode overwrite_mode;
 
-  gboolean enable_uncompressed_cache;
-  gboolean disable_fsync;
+  gboolean enable_uncompressed_cache;  /* Deprecated */
+  gboolean enable_fsync;  /* Deprecated */
   gboolean process_whiteouts;
   gboolean no_copy_fallback;