sysroot: Add ostree_sysroot_get_fd()

This way external programs like rpm-ostree can do fd-relative
operations on the deployment directories, like inspecting the RPM
database.

Closes: https://github.com/GNOME/ostree/pull/91

src/libostree/ostree-sysroot.h

@@ -39,6 +39,8 @@ OstreeSysroot* ostree_sysroot_new_default (void);
 
 GFile *ostree_sysroot_get_path (OstreeSysroot *self);
 
+int ostree_sysroot_get_fd (OstreeSysroot *self);
+
 gboolean ostree_sysroot_load (OstreeSysroot  *self,
                               GCancellable   *cancellable,
                               GError        **error);