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
Merge pull request #55 from daniel-murray/topic#sentrynobuild

Issue #8

if client builds sentry return, else continue