SIGHUP causes debug info in the error log and reload of
logs/privileges/tables/etc. The server should only do it when
a user intentionally sends SUGHUP, not when a parent terminal gets
disconnected or something.

In particular, not ignoring kernel SIGHUP causes FLUSH PRIVILEGES
at some random point during non-systemd Debian upgrades (Debian
restarts mysqld, debian-start script runs mysql_upgrade in the background,
postinit script ends and kernel sends SIGHUP to all background processes
it has started). And during mysql_upgrade privilege tables aren't
necessarily ready to be reloaded.