epicsEventDestroy errors when exiting IOCs with model-1 drivers

Soft IOCs with model-1 motor drivers don't (always?) exit cleanly.  The following error occurs and a `ctrl+c` is necessary to kill the IOC.
```
epics> exit
epicsEventDestroy: pthread_mutex_destroy failed: Device or resource busy
```
The original discussion of the issue can be found here: https://github.com/epics-motor/motorNewport/issues/13