Do not enable tx-carrier for non-interactive --file sessions

The logic that determined whether to 'block_input' was backwards with
respect to non-interactive --file sessions, such that using --file
*enabled* the tx-carrier behavior when it should have disabled it.
This went unnoticed because on reasonably fast systems, reading the
input data from the file never induce the timeout anyway -- only very
slow machines reveal the problem: long gaps of 'mark' between each
frame, as if each input character was slowly typed interactively.

Fixes: ecebf01 Add ability to output a carrier while waiting for data

src/minimodem.c

@@ -142,7 +142,7 @@ static void fsk_transmit_stdin(
 	{0, 0}						// it_value
     };
 
-    int block_input = !( tx_interactive && txcarrier );
+    int block_input = tx_interactive && !txcarrier;
     if ( block_input )
 	signal(SIGALRM, tx_stop_transmit_sighandler);