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/core/unit.c

@@ -4479,7 +4479,7 @@ static int operation_to_signal(KillContext *c, KillOperation k) {
                 return c->kill_signal;
 
         case KILL_KILL:
-                return SIGKILL;
+                return c->final_kill_signal;
 
         case KILL_ABORT:
                 return SIGABRT;