Support for old 32-bit machines

This fixes a bug preventing files > 4GiB being copied on 32-bit machines.

Improved searching for vhd parents

sparse_dd will now search the directory of the src vhd for the parents, rather than the current working directory of the process.

Drop in replacement for XenServer sparse_dd

The sparse_dd tool here

• uses O_DIRECT to avoid polluting caches
• out-of-heap buffers via Cstruct.t so we don't pollute the major heap or perform unnecessary GC
• avoids making extra data copies
• allows SSL to be hard-wired on or off via the config file (ie without recompile)
• avoids piping through client stunnel when SSL is on
• has experimental bypass-tapdisk-for-{read,write} options in the config file