checkout: Add an option to require hardlinks

I've seen a few people hit this and wonder why checkouts are slow/take
space.  Really, ensuring this happens is the *point* of OSTree.
Physical copies should be a last resort fallback for very unusual
situations (one of those is rpm-ostree checking out the db since
librpm doesn't know how to read from libostree).

Even I hit the fact that `/var` is a mountpoint disallowing hardlinks
with `/ostree` once and was confused.  =)

Add this to the rofiles-fuse test case because it creates a mount
point.

Closes: #368
Approved by: jlebon

src/libostree/ostree-repo.h

@@ -750,7 +750,8 @@ typedef struct {
   guint enable_uncompressed_cache : 1;
   guint disable_fsync : 1;
   guint process_whiteouts : 1;
-  guint reserved : 29;
+  guint no_copy_fallback : 1;
+  guint reserved : 28;
 
   const char *subpath;