Rework x/y-modem transfer command

Remove ctrl-t X optin and instead introduce submenu to ctrl-t x option
for picking which xmodem protocol to use.

src/xymodem.h

@@ -0,0 +1,30 @@
+/*
+ * tio - a simple serial terminal I/O tool
+ *
+ * Copyright (c) 2014-2024  Martin Lund
+ *
+ * This program is free software; you can redistribute it and/or
+ * modify it under the terms of the GNU General Public License
+ * as published by the Free Software Foundation; either version 2
+ * of the License, or (at your option) any later version.
+ *
+ * This program is distributed in the hope that it will be useful,
+ * but WITHOUT ANY WARRANTY; without even the implied warranty of
+ * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+ * GNU General Public License for more details.
+ *
+ * You should have received a copy of the GNU General Public License
+ * along with this program; if not, write to the Free Software
+ * Foundation, Inc., 51 Franklin Street, Fifth Floor, Boston, MA
+ * 02110-1301, USA.
+ */
+
+typedef enum {
+    XMODEM_1K,
+    XMODEM_CRC,
+    YMODEM,
+} modem_mode_t;
+
+extern char key_hit;
+
+int xymodem_send(int sio, const char *filename, modem_mode_t mode);