Report write() and ioctl() errors encountered when writing GPT or MBR to disk.

Report write() and ioctl() errors encountered when writing GPT or MBR to disk.

Consolidate GPT/MBR read()/write() operations into DISK_writesectors() and
DISK_readsectors(), producing clearer logic and consistent handling of i/o and
errors.

Add some DPRINTF() and #ifdef DEBUG sections to allow more detailed error
reporting when desired.