- changed autoloading of *.deh lumps: it is now controlled by a CVAR with 3 options: never load *.deh lumps, load all of them or only load the last one. *.deh loading is disabled by default.
- Use a more conformant C++ preprocessor macro identifier for Unix systems.

The 'unix' identifier isn't defined when '-std' is passed to the compiler (tested with gcc and clang), so use '__unix__' which is well enough documented.
Merge remote-tracking branch 'origin/maint' into maint