core: add support for RestartKillSignal= to override signal used for restart jobs

v2:
- if RestartKillSignal= is not specified, fall back to KillSignal=. This is necessary
  to preserve backwards compatibility (and keep KillSignal= generally useful).

test/TEST-23-TYPE-EXEC/testsuite.sh

@@ -19,6 +19,14 @@ systemd-run --unit=four -p Type=exec /bin/sleep infinity
 ! systemd-run --unit=five -p Type=exec -p User=idontexist /bin/sleep infinity
 ! systemd-run --unit=six -p Type=exec /tmp/brokenbinary
 
+systemd-run --unit=seven -p KillSignal=SIGTERM -p RestartKillSignal=SIGINT -p Type=exec /bin/sleep infinity
+# Both TERM and SIGINT happen to have the same number on all architectures
+test $(systemctl show --value -p KillSignal seven.service) -eq 15
+test $(systemctl show --value -p RestartKillSignal seven.service) -eq 2
+
+systemctl restart seven.service
+systemctl stop seven.service
+
 systemd-analyze log-level info
 
 echo OK > /testok

test/fuzz/fuzz-unit-file/directives.service

@@ -185,6 +185,7 @@ RequiresMountsFor=
 Requisite=
 Restart=
 RestartForceExitStatus=
+RestartKillSignal=
 RestartPreventExitStatus=
 RestartSec=
 ReusePort=