Add support for NTVDMx64

Yes, you can run 16-bit DOS doors on 64-bit (x64) Windows now.

Install NTVDMx64 (http://www.columbia.edu/~em36/ntvdmx64.html, it's not as onerous as it sounds) and re-enable DOS program support in SBBS (i.e. make sure "NO_DOS" is not in your sbbs.ini [bbs] Options value) and voila: DOS doors work.

This change adds an empty init routine to sbbsexec.dll since older versions of NTVDM (which NTVDMx64 is based on) required one. Also, the sbbsexec.dll should be located in your Synchronet "exec" directory when using NTVDMx64 (in addition to or instead of your Windows/System32 directory).