68000: Fix the stop instruction

When waking up from the stopped state due to an interrupt, the
CPU executes the next instruction before executing the interrupt.
Instead the interrupt should be executed immediately.
wd179x: Fix data flushing on mark detection during read track

During the 'read track' command, when a mark is detected the
current data register is output, but only if it contains valid
data.
wd179x: Use more tolerant command decoding
8086: Set the E86_CPU_PUSH_FIRST flag for the 80286

The 80286 and later CPUs push the unmodified value of SP. The ability
to do this was already implemented, but the relevant flag was not
set when the CPU type was set to 80286.
pri: Properly skip non-existing tracks in all operations
psi: Add new option '-D' to dump extra data

The extra data dumped depends on the input format. For now only the
STX track images are supported.
68901: Clear the timer reset bit when writing the control register
wd179x: Reset DRQ when a new command is started
wd179x: Report the physical cylinder during reads
psi/stx: Adjust the sector position correctly
atari: Make the keypad behave more logically

The keypad now maps to the cursor keys by default.
6850: Reset IRQ when the data register is read or written
68901: Split 32 bit accesses into 16 bit accesses
68901: Fix masking of control register bits
68901: Improve the interrupt logic

There's still a lot of guessing here, but this patch seems
to improve things somewhat.