Every SPI programmer driver had its own completely different chip write
implementation, and all of them were insufficiently commented.
Create spi_write_chunked as a copy of spi_read_chunked and convert all
SPI programmers to use it.
No functional changes except:
- Bus Pirate uses 12 Byte writes instead of 8 Byte writes
- SB600 uses 5 Byte writes instead of 1 Byte writes

Should work. Not for 0.9.1, but since I had that thing on my disk
anyway, I figured I'd post it to the list in case someone wants to
experiment with partial writes.
Tests appreciated.