Make final kill signal configurable

Usecase is to allow changing the final kill from SIGKILL to SIGQUIT which
should create a core dump useful for debugging why the service didn't stop
with the SIGTERM

src/shared/bus-unit-util.c

@@ -1189,7 +1189,7 @@ static int bus_append_kill_property(sd_bus_message *m, const char *field, const
 
                 return bus_append_parse_boolean(m, field, eq);
 
-        if (streq(field, "KillSignal"))
+        if (STR_IN_SET(field, "KillSignal", "FinalKillSignal"))
 
                 return bus_append_signal_from_string(m, field, eq);