Handle SIGTERM and SIGINT for clean exit

[mg-dev25]

Handle SIGTERM and SIGINT for clean exit

Adds signal handlers for SIGTERM and SIGINT using the existing sa_usr2() handler. Follows the feedback on PR #314 about reusing existing handlers.

What changed

Two lines added to the signal handler setup:

signal (SIGTERM, sa_usr2);  /* Clean exit on SIGTERM */
signal (SIGINT, sa_usr2);   /* Clean exit on SIGINT (Ctrl+C) */

The existing sa_usr2() handler exits with YAD_RESPONSE_CANCEL, which is the appropriate response for interrupted dialogs.

Why

Scripts embedding YAD dialogs need to properly terminate them when interrupted or receiving signals from process managers. Without this, SIGTERM and SIGINT would use the default behavior (immediate termination) instead of the clean exit path.

Testing

# Start dialog and send SIGTERM
yad --text="Test" &
kill $!
echo "Exit code: $?"  # Should be 1 (CANCEL)

# Start dialog and press Ctrl+C
yad --text="Test"  # Press Ctrl+C, should exit cleanly