Fix rsync exit handling in create mode.

The logic in handle_rsync_exit() is horribly broken in case dss is
run in create mode and the rsync process terminates unsuccessfully.

First we claim to restart rsync, which is wrong. Next we call the
post-create hook despite the documentation says that this hook is
only run on *successful* termination. Finally, we dereference a NULL
pointer to print the path of the snapshot.

Fortunately, all three issues are easy to fix by special casing create
mode in handle_rsync_exit().