Added exit code definitions to systemd service files (fixes #1324)

etc/linux-systemd/system/[email protected]

@@ -9,6 +9,9 @@ EnvironmentFile=-/etc/default/syncthing
 Environment=STNORESTART=yes
 ExecStart=/usr/bin/syncthing ${STARGS}
 Restart=on-failure
+RestartPreventExitStatus=1
+SuccessExitStatus=2
+RestartForceExitStatus=3 4
 
 [Install]
 WantedBy=multi-user.target

etc/linux-systemd/user/syncthing.service

@@ -7,6 +7,9 @@ EnvironmentFile=-%h/.config/syncthing/environment
 Environment=STNORESTART=yes
 ExecStart=/usr/bin/syncthing ${STARGS}
 Restart=on-failure
+RestartPreventExitStatus=1
+SuccessExitStatus=2
+RestartForceExitStatus=3 4
 
 [Install]
 WantedBy=cmdline.target