Add seeking code to sample.

Borrowed seeking code from Roland Gustafsson's RW18 implementation.

I also decreased the machine code reserve by 5.5k to make room for
more source code.
Sample can now write nibble image to disk.

I added low level drive writing code from the Format sample in "Beneath
Apple ProDOS" by Don Worth and Pieter Lechner".  With this code
inplace I was able to successfully write out a couple of nibble disk
images to floppy and boot from them.

I did have to init the floppy drive at the beginning of each track
since the reads on the real hardware which used ADTPro's VDRIVE
support would cause the drive for the low level write to be turned
off.

I dropped the initial track value down from half-track 160 to
half-track 80.  This bangs it against the stop much less and makes
it sounds more like the reboot path pulling out the head.