Use the new StripCtrlChars for terminal-based auth prompts.

SSH authentication prompts (passwords, passphrases and keyboard-
interactive) were previously sanitised to remove escape sequences by
the simplistic sanitise_term_data() in utils.c. Now they're fed
through the new mode of StripCtrlChars instead, which means they
should permit printable Unicode (if the terminal is in UTF-8 mode)
while still disallowing escape sequences. Hopefully this will be a
usability improvement to everyone whose login prompts are in a
language not representable in plain ASCII.

terminal.h

@@ -100,6 +100,9 @@ struct terminal_tag {
     termchar basic_erase_char, erase_char;
 
     bufchain inbuf;		       /* terminal input buffer */
+    bufchain_sink inbuf_bs;
+    StripCtrlChars *inbuf_scc;
+
     pos curs;			       /* cursor */
     pos savecurs;		       /* saved cursor position */
     int marg_t, marg_b;		       /* scroll margins */