Ensure that string type records are null terminated

This also ensures that writing more than 39 characters to a string record
will fail with a "string too long" error.

softioc/device.py

@@ -267,7 +267,7 @@ def _value_to_epics(self, value):
         # Value being written must be a string, and will be automatically null
         # terminated where possible.
         result = self._ctype_()
-        result.value = value.encode()
+        result.value = value.encode() + b'\0'
         return result
 
     def _epics_to_value(self, epics):