preserve_exit: Ignore SIGHUP

closes #9274
ref #9028

If stdin closed then read_error_exit calls preserve_exit. Handling
SIGHUP during preserve_exit would cause a premature teardown, and
conflicts with e.g. ui_bridge_stop which waits for TUI to teardown.

Vim ignores SIGHUP in its prepare_to_exit and getout_preserve_modified
routines:

    /* Ignore SIGHUP, because a dropped connection causes a read error, which
     * makes Vim exit and then handling SIGHUP causes various reentrance
     * problems. */
    signal(SIGHUP, SIG_IGN);

src/nvim/ui_bridge.c

@@ -117,6 +117,7 @@ static void ui_bridge_stop(UI *b)
     if (stopped) {  // -V547
       break;
     }
+    // TODO(justinmk): Remove this. Use a cond-wait above. #9274
     loop_poll_events(&main_loop, 10);  // Process one event.
   }
   uv_thread_join(&bridge->ui_thread);