vt: add modifier support to cursor keys

vt: add modifier support to cursor keys

Generate xterm-style CSI sequences with modifier parameters for arrow
keys when Shift, Alt, or Ctrl are held. For example, Shift+Up produces
ESC [ 1 ; 2 A instead of plain ESC [ A.

The modifier encoding follows the standard xterm convention:
  mod = 1 + (shift ? 1 : 0) + (alt ? 2 : 0) + (ctrl ? 4 : 0)

When no modifiers are pressed, the original behavior is preserved.

Explicit keymap bindings for modified cursor keys (e.g., "shift keycode
103 = Find") take precedence over this automatic modifier encoding.

Signed-off-by: Nicolas Pitre <npitre@baylibre.com>
Link: https://patch.msgid.link/20260203045457.1049793-2-nico@fluxnic.net
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>