Low: fencer: handle malformed manual confirmation requests better

Rename stonith_manual_ack() to fenced_handle_manual_confirmation(), and move
more of the manual confirmation handling in handle_request() into it, for
better code isolation. This will also make planned changes easier.

The one behavioral difference is that a failure of initiate_remote_stonith_op()
will now be ignored rather than segmentation fault trying to dereference NULL.