track *last* hidden's class
* when a new hidden is selected, store the last hidden's class
* this prevents the new hidden selection overwriting the choice
* then use g_lastHiddenSavedClass to restore the last hidden's
  class after the new hidden's class has been stored
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
Merge pull request #54 from daniel-murray/topic#hiddenlastclass

Potential resolution of Issue #49

track *last* hidden's class