Replaced IMG handler with one that now proxies to the 86F handler;
Applies the remaining Mainline PCem speedup commit;
Fixed the National Semiconductors PC87306 Super I/O Chip's serial port IRQ assignment;
DMF images are now loaded with the correct sector interleave, improving read/write speed;
XDF images are now loaded in a way that emulates the real order of the sectors on the track, improving read/write speed;
Added 16-bit physical memory read/write routines (mem_phys_readw, mem_phys_writew) and modified the 16-bit DMA code to use them instead of two 8-bit reads/writes.
