More big endian fixes for SP and MP, fixed G_ActivateBehavior to work with older compilers

I changed (bSID != -1) to (bSID != (bState_t)-1) in G_ActivateBehavior. Compiled with older GCC versions (<=4.4) the former expression always evaluated true, even when the value of bSID was -1. This lead to some sneaky bugs, for example the spawnscripts weren't executed for NPCs.