input: don't return zero when some function is completely unbound

Returning a zero would mean returning a ^Space -- the keycode that
by default is bound to the Next-Word function.  So, unbinding the
keys to which the modified Arrow keys are hard bound would cause
the latter ones to mysteriously jump to the next word.

This partially fixes https://savannah.gnu.org/bugs/?49058.

src/winio.c

@@ -495,38 +495,38 @@ int parse_kbinput(WINDOW *win)
 	return ERR;
 
     if (retval == controlleft)
-	return sc_seq_or(do_prev_word_void, 0);
+	return sc_seq_or(do_prev_word_void, controlleft);
     else if (retval == controlright)
-	return sc_seq_or(do_next_word_void, 0);
+	return sc_seq_or(do_next_word_void, controlright);
     else if (retval == controlup)
-	return sc_seq_or(do_prev_block, 0);
+	return sc_seq_or(do_prev_block, controlup);
     else if (retval == controldown)
-	return sc_seq_or(do_next_block, 0);
+	return sc_seq_or(do_next_block, controldown);
 #ifndef NANO_TINY
     else if (retval == shiftcontrolleft) {
 	shift_held = TRUE;
-	return sc_seq_or(do_prev_word_void, 0);
+	return sc_seq_or(do_prev_word_void, shiftcontrolleft);
     } else if (retval == shiftcontrolright) {
 	shift_held = TRUE;
-	return sc_seq_or(do_next_word_void, 0);
+	return sc_seq_or(do_next_word_void, shiftcontrolright);
     } else if (retval == shiftcontrolup) {
 	shift_held = TRUE;
-	return sc_seq_or(do_prev_block, 0);
+	return sc_seq_or(do_prev_block, shiftcontrolup);
     } else if (retval == shiftcontroldown) {
 	shift_held = TRUE;
-	return sc_seq_or(do_next_block, 0);
+	return sc_seq_or(do_next_block, shiftcontroldown);
     } else if (retval == shiftaltleft) {
 	shift_held = TRUE;
-	return sc_seq_or(do_home, 0);
+	return sc_seq_or(do_home, shiftaltleft);
     } else if (retval == shiftaltright) {
 	shift_held = TRUE;
-	return sc_seq_or(do_end, 0);
+	return sc_seq_or(do_end, shiftaltright);
     } else if (retval == shiftaltup) {
 	shift_held = TRUE;
-	return sc_seq_or(do_page_up, 0);
+	return sc_seq_or(do_page_up, shiftaltup);
     } else if (retval == shiftaltdown) {
 	shift_held = TRUE;
-	return sc_seq_or(do_page_down, 0);
+	return sc_seq_or(do_page_down, shiftaltdown);
     }
 #endif
 
@@ -539,13 +539,13 @@ int parse_kbinput(WINDOW *win)
 	/* Is Ctrl being held? */
 	if (modifiers & 0x04) {
 	    if (retval == KEY_UP)
-		return sc_seq_or(do_prev_block, 0);
+		return sc_seq_or(do_prev_block, controlup);
 	    else if (retval == KEY_DOWN)
-		return sc_seq_or(do_next_block, 0);
+		return sc_seq_or(do_next_block, controldown);
 	    else if (retval == KEY_LEFT)
-		return sc_seq_or(do_prev_word_void, 0);
+		return sc_seq_or(do_prev_word_void, controlleft);
 	    else if (retval == KEY_RIGHT)
-		return sc_seq_or(do_next_word_void, 0);
+		return sc_seq_or(do_next_word_void, controlright);
 	}
 
 #ifndef NANO_TINY
@@ -556,13 +556,13 @@ int parse_kbinput(WINDOW *win)
 	/* Are both Shift and Alt being held? */
 	if ((modifiers & 0x09) == 0x09) {
 	    if (retval == KEY_UP)
-		return sc_seq_or(do_page_up, 0);
+		return sc_seq_or(do_page_up, shiftaltup);
 	    else if (retval == KEY_DOWN)
-		return sc_seq_or(do_page_down, 0);
+		return sc_seq_or(do_page_down, shiftaltdown);
 	    else if (retval == KEY_LEFT)
-		return sc_seq_or(do_home, 0);
+		return sc_seq_or(do_home, shiftaltleft);
 	    else if (retval == KEY_RIGHT)
-		return sc_seq_or(do_end, 0);
+		return sc_seq_or(do_end, shiftaltright);
 	}
 #endif
     }
@@ -633,13 +633,13 @@ int parse_kbinput(WINDOW *win)
 #endif
 	case DEL_CODE:
 	    if (ISSET(REBIND_DELETE))
-		return sc_seq_or(do_delete, keycode);
+		return sc_seq_or(do_delete, KEY_DC);
 	    else
 		return KEY_BACKSPACE;
 #ifdef KEY_SIC
 	/* Slang doesn't support KEY_SIC. */
 	case KEY_SIC:
-	    return sc_seq_or(do_insertfile_void, keycode);
+	    return sc_seq_or(do_insertfile_void, KEY_IC);
 #endif
 #ifdef KEY_SBEG
 	/* Slang doesn't support KEY_SBEG. */
@@ -667,7 +667,7 @@ int parse_kbinput(WINDOW *win)
 #endif
 	/* Slang doesn't support KEY_SUSPEND. */
 	case KEY_SUSPEND:
-	    return sc_seq_or(do_suspend_void, 0);
+	    return sc_seq_or(do_suspend_void, KEY_SUSPEND);
 #endif
 #ifdef PDCURSES
 	case KEY_SHIFT_L: