Various minor fixes/improvements (#239)

Includes:
- Making "rpm_data_free" a public method and using it where appropriate
  - Includes fixing memory leak in epics demo application
    (note Valgrind indicates that there are more...)
- Fix typo in cov.c
- Allow NULL max_apdu parameter to address_get_by_device
- Add missing include to s_cov.h

apps/readpropm/main.c

@@ -142,21 +142,8 @@ static void My_Read_Property_Multiple_Ack_Handler(uint8_t *service_request,
         if (len > 0) {
             while (rpm_data) {
                 rpm_ack_print_data(rpm_data);
-                rpm_property = rpm_data->listOfProperties;
-                while (rpm_property) {
-                    value = rpm_property->value;
-                    while (value) {
-                        old_value = value;
-                        value = value->next;
-                        free(old_value);
-                    }
-                    old_rpm_property = rpm_property;
-                    rpm_property = rpm_property->next;
-                    free(old_rpm_property);
-                }
-                old_rpm_data = rpm_data;
-                rpm_data = rpm_data->next;
-                free(old_rpm_data);
+                rpm_data = rpm_data_free(rpm_data);
+                
             }
         } else {
             fprintf(stderr, "RPM Ack Malformed! Freeing memory...\n");