restart doesn't test for syntax errors

Tested openssh on bionic and groovy, same issue.

The switch to systemd lost the ability to do a sanity check on the config file (via sshd -t) before attempting to restart sshd. This was originally bug #624361 in the SySV days, fixed in the initscript back then.

The sysv script still does it, but it's not used anymore:
 restart)
        check_privsep_dir
        check_config
        log_daemon_msg "Restarting OpenBSD Secure Shell server" "sshd" || true

And:
check_config() {
    if [ ! -e /etc/ssh/sshd_not_to_be_run ]; then
        /usr/sbin/sshd $SSHD_OPTS -t || exit 1
    fi
}

The systemd service file has only ExecStartPre, which doesn't let it start if there is an error, but will happily stop it:
[Unit]
Description=OpenBSD Secure Shell server
After=network.target auditd.service
ConditionPathExists=!/etc/ssh/sshd_not_to_be_run

[Service]
EnvironmentFile=-/etc/default/ssh
ExecStartPre=/usr/sbin/sshd -t
ExecStart=/usr/sbin/sshd -D $SSHD_OPTS
ExecReload=/usr/sbin/sshd -t
ExecReload=/bin/kill -HUP $MAINPID
...

Example:
# sshd -t
# systemctl restart sshd
# telnet localhost 22
Trying 127.0.0.1...
Connected to localhost.
Escape character is '^]'.
SSH-2.0-OpenSSH_7.6p1 Ubuntu-4ubuntu0.3
^]
telnet> quit
Connection closed.

# echo "syntax error" >> /etc/ssh/sshd_config
# sshd -t
/etc/ssh/sshd_config: line 123: Bad configuration option: syntax
/etc/ssh/sshd_config: terminating, 1 bad configuration options

# systemctl restart sshd
Job for ssh.service failed because the control process exited with error code.
See "systemctl status ssh.service" and "journalctl -xe" for details.

# telnet localhost 22
Trying 127.0.0.1...
telnet: Unable to connect to remote host: Connection refused
#