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.
Changes to the memory manager reduced by 120MB for machines with CPU < 80386.
Changes to the 86F diskette data processor (required by all floppy controllers) reduced by 330M.
Test machine OpenXT now runs at 240MB, instead of 680MB.