Making --inplace and --sparse compatible

[Perry Hutchison]

It seems to me that it should be possible to make --inplace and
--sparse compatible.

The manpage says

    -S, --sparse
           ... Conflicts with --inplace because it's not
           possible to overwrite data in a sparse fashion.

but I think it _is_ possible to do that, provided the receiving
side opens the dest file read-write instead of write-only.

When writing within the existing length of the dest, it should be
possible -- when processing a src block which is all zero and thus
would be seeked over instead of written when writing sparsely to a
file being newly created -- to read that block from the dest and
refrain from writing it back if it is already zero in the dest file.
(Of course, if it is _not_ all zero in the dest, the non-zero bytes
would have to be overwritten with zeros.)

Granted this approach will not introduce sparseness into an existing
non-sparse dest, but it at least will preserve any sparseness which
already exists in the dest.

I'm currently using 3.0.7, but I think the same issue still exists
in 3.1.0 since there's no mention of such a change in the release
notes and the 3.1.0 manpage still documents the same limitation.

Thoughts?