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

src/bacnet/basic/service/h_rpm_a.c

@@ -293,7 +293,7 @@ void rpm_ack_print_data(BACNET_READ_ACCESS_DATA *rpm_data)
  * @param rpm_data - #BACNET_READ_ACCESS_DATA
  * @return RPM data from the next element in the linked list
  */
-static BACNET_READ_ACCESS_DATA *rpm_data_free(BACNET_READ_ACCESS_DATA *rpm_data)
+BACNET_READ_ACCESS_DATA *rpm_data_free(BACNET_READ_ACCESS_DATA *rpm_data)
 {
     BACNET_READ_ACCESS_DATA *old_rpm_data = NULL;
     BACNET_PROPERTY_REFERENCE *rpm_property = NULL;