MSVC-compatible use of #pragma pack.
Fixed potential crash in Sys_open.
Use MSVC intrinsics for byte swapping.
Ensure use of winsock2 (instead of winsock).
Set appropriate architecture #defines.
Fixed various other compile warnings/errors.
Other bugs fixed:
- CD-ROM media are polled and now can be changed without rebooting
- Buffer overflow, memory leak and extra wait in CD-ROM ejection code
