drbdsetup X resume-io should be usable to resume IO [Bugz 256]

When IO gets frozen due to a broken fence-peer script, the user
should be able to thaw IO by the resume-io command.

Signed-off-by: Philipp Reisner <philipp.reisner@linbit.com>
Signed-off-by: Lars Ellenberg <lars.ellenberg@linbit.com>