sig_handler: use _exit() for SIGSEGV, keep exit() for clean shutdown

[Pace17881]

`exit()` is not async-signal-safe (POSIX) and runs the `atexit(cleanup)` handler, which tears down the X11 connection, unix socket, and devices. After a real segfault, process state may be corrupted, and running that same teardown code from within the signal handler could itself crash or hang - somewhat counterproductive for a handler whose whole point is to "exit gracefully".

This only changes the `SIGSEGV` case to call `_exit()` directly. `SIGINT`/`SIGTERM` keep using `exit()` as before, so normal shutdown still runs `cleanup()` and releases everything properly - only the segfault path skips it.

As a side effect, this also resolves the `-Wimplicit-fallthrough` warning under `-Wextra`, since `SIGSEGV` no longer falls through into the other cases (found while looking at the codebase with stricter warnings enabled, unrelated to the fix itself).

Small, self-contained change - happy to adjust if you'd prefer a different approach.

[jtsiomb]

Correct me if I'm wrong, but doesn't that make attempting to handle SIGSEGV completely pointless?

It was an attempt to maybe get the chance to clean some things up before exiting, if the situation allows it. But with your change we'll just exit immediately without any cleanup, which is what the default SIGSEGV handler would do anyway.

[Pace17881]

Sorry for the noise — you're right, I hadn't considered that trading a chance at cleanup for a guaranteed no-op isn't actually an improvement. Closing this.