Improve the port and catch it up with modern reality a bit:
- Fix a runtime crash (segfault) due to `MasterDirectory dirbuf' if-block local variable being used after it went out of scope by making it local to the LoadPicture() function instead
- Add the HEXEN option which brings better Hexen support to Yadex (duh). Unfortunately, it depends on the SEARCH patch and conflicts with the LAPTOP_KEYS patch (which is the reason why I didn't introduce it years ago: we did not have IMPLIES/PREVENTS back then, but we do now)
- Building with 3D Render option requires USE_CXXSTD=c++98 due to infamous "in-class initializer for static data member of type 'const double' requires 'constexpr' specifier" error in `src/r_render.cc'. Actually, we could get away with c++03, but GCC 4.2, which is still used on e.g. powerpc and sparc64, does not support it. Enable it unconditionally so the port is always built against the same C++ standard regardless of the selected options
- Reword option descriptions to avoid using verbs, per our conventions