The launcher and the watchdog of flexisip are removed and replaced by
the services management of SystemD that provides a watchdog. The
behavior of a Flexisip service is configured in the SystemD unit
files. By default, a service notifies SystemD once startup is
complete, using `Type=notify`. The SystemD watchdog is then enabled,
and the service's main loop periodically notifies the watchdog to
confirm that the process is alive and not blocked. The watchdog
timeout is configured in the service's unit file by `WatchdogSec` (10s
by default), while the main loop notification interval is set via the global
parameter `watchdog-notify-interval` (1s by default).

Flexisip now uses `Restart=on-failure` in its SystemD service units by
default. This ensures that if a service crashes, exits with a non-zero
status or fails to notify the watchdog before the timeout, SystemD
automatically restarts it. SystemD also restarts if the
RESTART_EXIT_CODE is received (`RestartForceExitStatus=5`). There is
a delay `RestartSec` before restarting the service. If the startup
fails, the service is not restarted.

The option `--daemon` in command line is not used anymore to start a
service.

The option `--disable-stdout` in command line is added to disable the
display of logs in standard output. This option is used in the
configuration of the Flexisip services for SystemD.

The global parameter `auto-respawn` is not used anymore.

The library libsystemd is used to notify SystemD, if
ENABLE_SYSTEMD=ON (default on linux).

Update mainTester : the test starts a server with all services and
check that the proxy is properly initiated by sending a request with
cli. Then the server is stopped and the test checks that it exited
cleanly.

Update docker images with library for SystemD.

Update readme and changelog.