Make the default Prestel ID 0 bytes long.

CCl4 sends an ENQ character in the login screen, and sends an ENQ
which will auto-fill your username.  Also, if you enter six zeros
into the username, it will take you to the password field.

With ten zeros, this means it's impossible to log in to CCl4.

src/conio/cterm.c

@@ -4894,7 +4894,7 @@ struct cterminal* cterm_init(int height, int width, int xpos, int ypos, int back
 	if (cterm->emulation == CTERM_EMULATION_PRESTEL) {
 		cterm->cursor = _NOCURSOR;
 		SETCURSORTYPE(cterm->cursor);
-		memcpy(cterm->prestel_data[0], "0000000000??????", PRESTEL_MEM_SLOT_SIZE);
+		memcpy(cterm->prestel_data[0], "????????????????", PRESTEL_MEM_SLOT_SIZE);
 		memcpy(cterm->prestel_data[1], ":2:?4967?89:1???", PRESTEL_MEM_SLOT_SIZE);
 		memcpy(cterm->prestel_data[2], ":1632?123?456???", PRESTEL_MEM_SLOT_SIZE);
 		memcpy(cterm->prestel_data[3], ";???????????????", PRESTEL_MEM_SLOT_SIZE);