64-bit Windows builds

It seems to be the time to start looking into building 64-bit (x64) executables for Windows.

• sbbsctrl (built with C++Builder 6) definitely won't be moving to 64-bit as is
• 3rd party libs (libmozjs, cryptlib, sdl) might provide their own challenges

But the additional Windows security and address space available to 64-bit Windows programs might be important to some sysops, if not now, in the not too distant future.