[directfd] Update DF driver for bugs on multiple opens

[ghaerr]

Adds per-drive globals (amongst all the other globals) to fix problems identified by @Mellvik in Mellvik/TLVC#95 when a second DF device is opened prior to any I/O requests received for the first device (or vice-versa).

Other changes include:

• Don't auto-probe floppy on pre-AT systems
• Group "dual" globals together at top of driver so easy to see
• Don't display first auto-probe message (as either result or next probe is always displayed)
• Changed "implied seek" printk to debug only
• Changed "auto-detected" message to just display floppy type
• Always disable interrupts before accessing 8237 DMA controller (for shared flip-flop, actually just removes FIXME comment)
• Fixes small bug disabling next request when invalid FDC command seen (wouldn't happen anyways)