Rename MFM (and RLL) to ST506 all over. Config files will be updated.
XT-IDE driver rework.
Cleanups in the HDC drivers.
This commit requires a new romset, HDC roms have been moved!
Converted AT NVR to be a device, so it gets auto-closed.
Added new machine_close() function which calls optional machine-custom close, updated machine table.
Small fixes here and there.
Re-wored the CDROM code to have a 'speeds' table instead of ugly hard-coding.
Moved the SIO files to a new sio/ folder.
Added the 'version.h'stuff, added Makefile rule to generate the Manifest file for Windows.
Re-worked the About dialog - it now includes more info.
