vt: add KT_CSI keysym type for modifier-aware CSI sequences

vt: add KT_CSI keysym type for modifier-aware CSI sequences

Add a new keysym type KT_CSI that generates CSI tilde sequences with
automatic modifier encoding. The keysym value encodes the CSI parameter
number, producing sequences like ESC [ <value> ~ or ESC [ <value> ; <mod> ~
when Shift, Alt, or Ctrl modifiers are held.

This allows navigation keys (Home, End, Insert, Delete, PgUp, PgDn) and
function keys to generate modifier-aware escape sequences without
consuming string table entries for each modifier combination.

Define key symbols for navigation keys (K_CSI_HOME, K_CSI_END, etc.)
and function keys (K_CSI_F1 through K_CSI_F20) using standard xterm
CSI parameter values.

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

Allowed CSI parameter values range from 0 to 99.

Note: The Linux console historically uses a non-standard double-bracket
format for F1-F5 (ESC [ [ A through ESC [ [ E) rather than the xterm
tilde format (ESC [ 11 ~ through ESC [ 15 ~). The K_CSI_F1 through
K_CSI_F5 definitions use the xterm format. Converting F1-F5 to KT_CSI
would require updating the "linux" terminfo entry to match. Navigation
keys and F6-F20 already use the tilde format and are fully compatible.

Signed-off-by: Nicolas Pitre <npitre@baylibre.com>
Link: https://patch.msgid.link/20260203045457.1049793-3-nico@fluxnic.net
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>