repo: Add a "force copy" flag to checkout

This is intended to be used for copying `/usr/etc` → `/etc` for
deployments.

A TODO here is to use `glnx_file_copy_at()` if the repo mode allows
it - then we'd use reflinks if available.

Closes: #804
Approved by: jlebon

src/libostree/ostree-repo.h

@@ -767,7 +767,8 @@ typedef struct {
   gboolean enable_fsync;  /* Deprecated */
   gboolean process_whiteouts;
   gboolean no_copy_fallback;
-  gboolean unused_bools[7];
+  gboolean force_copy; /* Since: 2017.6 */
+  gboolean unused_bools[6];
 
   const char *subpath;