SCP: Defer readline initialization until immediately before it will be used.

On some platforms, readline seems to behave in unexpected ways if another
file is already open when it is initialized.  Fix: #291

scp.c

@@ -7107,7 +7107,7 @@ static readline_func p_readline = NULL;
 typedef void (*add_history_func)(const char *);
 static add_history_func p_add_history = NULL;
 
-if (!initialized) {
+if (prompt && (!initialized)) {
     initialized = 1;
     void *handle;