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/s_cov.h

@@ -39,6 +39,7 @@
 #include "bacnet/bacenum.h"
 #include "bacnet/apdu.h"
 #include "bacnet/cov.h"
+#include "bacnet/npdu.h"
 
 #ifdef __cplusplus
 extern "C" {