etc: Add RestartSec=5 to linux-systemd units

Might help avoiding the 'Start request repeated too quickly.' error.

Fixes #6633, fixes #6634

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

@@ -7,6 +7,7 @@ After=network.target
 User=%i
 ExecStart=/usr/bin/syncthing -no-browser -no-restart -logflags=0
 Restart=on-failure
+RestartSec=5
 SuccessExitStatus=3 4
 RestartForceExitStatus=3 4
 

etc/linux-systemd/user/syncthing.service

@@ -5,6 +5,7 @@ Documentation=man:syncthing(1)
 [Service]
 ExecStart=/usr/bin/syncthing -no-browser -no-restart -logflags=0
 Restart=on-failure
+RestartSec=5
 SuccessExitStatus=3 4
 RestartForceExitStatus=3 4