Don't (ab)use the gen_server timeout mechanism for pinging the systemd
watchdog. Under certain conditions (e.g., the process receiving sys
messages), the gen_server timeout might not be triggered as expected.
Fixes#4054, fixes#4058,
Support systemd's watchdog feature and enable it by default in the unit
file, so that ejabberd is auto-restarted if the VM becomes unresponsive.
Also, set the systemd startup type to 'notify', so that startup of
followup units is delayed until ejabberd signals readiness. While at
it, also notify systemd of configuration reload and shutdown states.
Note: "NotifyAccess=all" is required as long as "ejabberdctl foreground"
runs the VM as a new child process, rather than "exec"ing it. This way,
systemd views the ejabberdctl process itself as the main service
process, and would discard notifications from other processes by
default.
