Hint to the compiler that these functions never return
A fatal-family function that can format its argument
Correct function name in fatal call
Improve handling of incomplete reads

a failure with EAGAIN should be tried again (though a timeout / select
would be appropriate?); a success with zero bytes read or written is
treated as a failure (is this correct?)
Remove unused code
Factor out nbd_decrypt_range
Implement network block device on Linux
Update the readme with information about mounting encryped zfs pools