Our docs claim that we set vm.max_map_count automatically. This is not
quite the case. The story is that on SysV init we set vm.max_map_count
each time the service starts, which is good. On systemd, we create a
sysctl.d conf file that sets vm.map_max_count, but this is only
meaningful if the system is rebooted after package install. This commit
modifies the post-install script so that we run systemd-sysctl so that
the vm.max_map_count change occurs after package install without a
reboot.

Relates #21507