if client builds sentry return, else continue

* this code is based on buildrestrict.sp
* we don't need to modify anything as we simply want to block all sentries
  when the command listener is active
* when buildrestrict identifies that the limit has been reached
  it simply returns Plugin_Handled, seeing as how we want 0 sentries
  we can just also return on sentry

* not sure if this will cost metal, probably not