Fix file truncation on save.

dd issues one call to read() per block, so using a large blocksize with
a count of 1 means it only calls read() once. Bash's tcp redirection
doesn't always seem to return a full buffer in this case (often
stopping at 32KB), which results in files being truncated.

Avoid this by using head -c. head will use a sensible buffer size
automatically and continue to call read() until exactly the number of
bytes requested have been read. This fixes issue #5.