simple sparse file support, fixes #256

[ThomasWaldmann]

Implemented sparse file support to remove this blocker for people backing up lots of
huge sparse files (like VM images). Attic could not support this use case yet as it would
have restored all files to their fully expanded size, possibly running out of disk space if
the total expanded size would be bigger than the available space.

Please note that this is a very simple implementation of sparse file support - at backup time,
it does not do anything special (it just reads all these zero bytes, chunks, deduplicates, compresses and
encrypts them as usual). At restore time, it detects chunks that are completely filled with zeros
and does a seek on the output file rather than a normal data write, so it creates a hole in
a sparse file. The chunk size for these all-zero chunks is currently 10MiB, so it'll create holes
of multiples of that size (depends also a bit on fs block size, alignment, previously written data).

Special cases like sparse files starting and/or ending with a hole are supported.

Please note that it will currently always create sparse files at restore time if it detects all-zero
chunks.

Also improved:
I needed a constant for the max. chunk size, so I introduced CHUNK_MAX (see also
existing CHUNK_MIN) for the maximum chunk size (which is the same as the chunk
buffer size).

Attic still always uses 10MiB chunk buffer size now, but it could be changed now more easily.

[ThomasWaldmann]

About the test failure see issue #304.

[ThomasWaldmann]

closing this pull request, seems unwanted.