Linux MS/TP Fixes (#1051)

* Linux MS/TP:
 * dlmstp_init:
   * Maximize thread priority if permitted

 * MSTP_Put_Receive/dlmstp_receive:
   * Use a buffer so incoming frames are not dropped during bursts

 * dlmstp_receive:
   * Make timeout optional

 * MSTP_Get_Reply:
   * Lock the mutex before using the PDU Queue
   * Iterate over all queued replies to find a match (instead of only checking the first)
   * Sleep for a millisecond before checking again to wait for the application provide a reply

 * RS485_Check_UART_Data:
   * Ensure waiter is initilized before using with select
   * Ensure bytes are read before adding to FIFO

TSM:
 * Add log when `datalink_send_pdu` fails for consistency with all other `datalink_send_pdu` calls

dlenv_maintenance_timer:
 * Fix Tx/Rx labels in MSTP statitistics print

* don't log on queued requests

src/bacnet/basic/tsm/tsm.c

@@ -299,6 +299,7 @@ bool tsm_get_transaction_pdu(
 void tsm_timer_milliseconds(uint16_t milliseconds)
 {
     unsigned i = 0; /* counter */
+    int bytes_sent = 0;
 
     BACNET_TSM_DATA *plist = &TSM_List[0];
 
@@ -314,13 +315,16 @@ void tsm_timer_milliseconds(uint16_t milliseconds)
                 if (plist->RetryCount < apdu_retries()) {
                     plist->RequestTimer = apdu_timeout();
                     plist->RetryCount++;
-                    datalink_send_pdu(
+                    bytes_sent = datalink_send_pdu(
                         &plist->dest, &plist->npdu_data, &plist->apdu[0],
                         plist->apdu_len);
                     DEBUG_PRINTF(
                         "invoke-id[%u] Retry %u of %u after %ums\n",
                         plist->InvokeID, plist->RetryCount, apdu_retries(),
                         plist->RequestTimer);
+                    if (bytes_sent <= 0) {
+                        debug_perror("invoke-id[%u] Failed to Send Retry");
+                    }
                 } else {
                     /* note: the invoke id has not been cleared yet
                        and this indicates a failed message: