Add state to string API function

Added a state to string public API function. There was already
a private function (`relay_chn_state_str`) that provides
this functionality. So this function has been renamed to
`relay_chn_state_to_str` and made publicly available.

Refs #1104, #1105 and closes #1108.

src/relay_chn_core.c

@@ -471,7 +471,7 @@ char *relay_chn_cmd_str(relay_chn_cmd_t cmd)
     }
 }
 
-char *relay_chn_state_str(relay_chn_state_t state)
+char *relay_chn_state_to_str(relay_chn_state_t state)
 {
     switch (state) {
         case RELAY_CHN_STATE_IDLE: