-Add a CVAR co_fixzerotags (disabled by default) to allow proper handling of moving sector line specials with a tag of 0, similar to ZDoom 1.23b33. Vanilla Doom behavior has been restored when the CVAR is disabled, rather than the previous ZDoom 1.22 behavior that caused issues when a moving sector is re-triggered (bug 981).

git-svn-id: http://odamex.net/svn/root/trunk@4115 b0bcef46-2027-0410-bd56-ab726e9a1d04