Added func_detail_blocker support to VBSP

Added support for the func_detail_blocker entity that was originally
introduced in L4D. It prevents detail props from being placed inside its
volume. See also:
https://developer.valvesoftware.com/wiki/Func_detail_blocker
Merge branch 'master' of https://github.com/wouterpleizier/source-sdk-2013