Allow restart for oneshot units

Picked up from https://github.com/systemd/systemd/pull/7474 , so
coauthored by @robermorales.

test/TEST-41-ONESHOT-RESTART/testsuite.sh

@@ -0,0 +1,33 @@
+#!/bin/bash
+set -ex
+set -o pipefail
+
+systemd-analyze log-level debug
+systemd-analyze log-target console
+
+# These three commands should succeed.
+! systemd-run --unit=one -p Type=oneshot -p Restart=on-failure /bin/bash -c "exit 1"
+
+sleep 5
+
+if [[ "$(systemctl show one.service -p NRestarts --value)" -le 0 ]]; then
+  exit 1
+fi
+
+TMP_FILE="/test-41-oneshot-restart-test"
+
+touch $TMP_FILE
+
+! systemd-run --unit=two -p StartLimitBurst=3 -p Type=oneshot -p Restart=on-failure -p ExecStart="/bin/bash -c \"printf a >>  $TMP_FILE\"" /bin/bash -c "exit 1"
+
+sleep 5
+
+if [[ $(cat $TMP_FILE) != "aaa" ]]; then
+  exit 1
+fi
+
+systemd-analyze log-level info
+
+echo OK > /testok
+
+exit 0