Limit carrier/idle generation for input with delays

For non-interactive output (writing to an audio file), generate the
idle output carrier for same duration that the input is idle.  Makes
this case generate an output .wav file of just over 1 second duration:

  { echo hello; sleep 1; echo world; } | minimodem --tx -f /tmp/xx.wav 300

src/minimodem.c

@@ -163,6 +163,14 @@ static void fsk_transmit_stdin(
         FD_ZERO(&fdset);
         FD_SET(fd, &fdset);
         struct timeval tv_idletimeout = { 0, 0 };
+
+	if ( !tx_interactive ) {
+	    // When stdin blocks we "emit idle tone", for a duration of
+	    // idle_carrier_usec.  If !tx_interactive (i.e. writing to an
+	    // audio file) make the select timeout the same duration.
+	    tv_idletimeout.tv_usec = idle_carrier_usec;
+	}
+
         if( block_input || select(fd+1, &fdset, NULL, NULL, &tv_idletimeout) )
         {
 	    n_read = read(fd, &buf, sizeof(buf));