selinux: add trigger for policy reload to refresh internal selabel cache

selinux: add trigger for policy reload to refresh internal selabel cache

Reload the internal selabel cache automatically on SELinux policy reloads so non pid-1 daemons are participating.

Run the reload function `mac_selinux_reload()` not manually on daemon-reload, but rather pass it as callback to libselinux.
Trigger the callback prior usage of the systemd internal selabel cache by depleting the selinux netlink socket via `avc_netlink_check_nb()`.

Improves: a9dfac21ec85 ("core: reload SELinux label cache on daemon-reload")
Improves: #13363