xinetd: Fix systemd service for systemd>237

SystemD 237 implements a stricted PIDfile handling that breaks several
daemons [1].

Change the way we start xinetd, mimicing other distros [2].

Fixes:
root@qt5122:~# journalctl -u xinetd
-- Logs begin at Fri 2018-05-25 14:33:29 UTC, end at Mon 2018-06-11 07:33:08 UTC. --
May 25 14:33:31 qt5122 systemd[1]: Starting Xinetd A Powerful Replacement For Inetd...
May 25 14:33:31 qt5122 systemd[1]: xinetd.service: Permission denied while opening PID file or unsafe symlink chain: /var/run/xinetd.pid
Jun 11 07:27:24 qt5122 systemd[1]: xinetd.service: Start operation timed out. Terminating.
Jun 11 07:27:24 qt5122 systemd[1]: xinetd.service: Failed with result 'timeout'.
Jun 11 07:27:24 qt5122 systemd[1]: Failed to start Xinetd A Powerful Replacement For Inetd.
Jun 11 07:31:38 qt5122 systemd[1]: Starting Xinetd A Powerful Replacement For Inetd...
Jun 11 07:31:38 qt5122 systemd[1]: xinetd.service: Permission denied while opening PID file or unsafe symlink chain: /var/run/xinetd.pid
Jun 11 07:33:08 qt5122 systemd[1]: xinetd.service: Start operation timed out. Terminating.
Jun 11 07:33:08 qt5122 systemd[1]: xinetd.service: Failed with result 'timeout'.
Jun 11 07:33:08 qt5122 systemd[1]: Failed to start Xinetd A Powerful Replacement For Inetd.

[1]: https://github.com/systemd/systemd/issues/8085
[2]: https://github.com/felixonmars/archlinux-community/commit/e49040636737fdb09031e75ec4d73a580f80676b

(From OE-Core rev: eea25f03e4c46cfe0d05df46f8f50e1389179c80)

Signed-off-by: Ricardo Ribalda Delgado <ricardo.ribalda@gmail.com>
Signed-off-by: Ross Burton <ross.burton@intel.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>

meta/recipes-extended/xinetd/xinetd/xinetd.service

@@ -3,11 +3,8 @@ Description=Xinetd A Powerful Replacement For Inetd
 After=syslog.target network.target
 
 [Service]
-Type=forking
-PIDFile=/var/run/xinetd.pid
 EnvironmentFile=-/etc/sysconfig/xinetd
-ExecStart=@SBINDIR@/xinetd -stayalive -pidfile /var/run/xinetd.pid "$EXTRAOPTIONS"
-ExecReload=@BASE_BINDIR@/kill -HUP $MAINPID
+ExecStart=@SBINDIR@/xinetd -dontfork "$EXTRAOPTIONS"
 
 [Install]
 WantedBy=multi-user.target