lib/core: Add a "break hardlink" API

This imports the code from rpm-ostree:
https://github.com/projectatomic/rpm-ostree/blob/9ff9f6c997d914cb7d97d6b59d8045ba64a1882c/src/libpriv/rpmostree-util.c#L742

I plan to use this for rofiles-fuse to implement
copyup: https://github.com/ostreedev/ostree/issues/1377

But it's just obviously generally useful for projects using
libostree I think.

Closes: #1378
Approved by: jlebon

src/libostree/ostree-core.h

@@ -438,6 +438,13 @@ gboolean ostree_checksum_file (GFile             *f,
                                GCancellable      *cancellable,
                                GError           **error);
 
+_OSTREE_PUBLIC
+gboolean ostree_break_hardlink (int               dfd,
+                                const char       *path,
+                                gboolean          skip_xattrs,
+                                GCancellable     *cancellable,
+                                GError          **error);
+
 /**
  * OstreeChecksumFlags:
  *