s3: libsmbclient: Add server-side copy support

s3: libsmbclient: Add server-side copy support

Introduce a new operation, splice, which copies data from one SMBCFILE
to another. Implement this operation using FSCTL_SRV_COPYCHUNK_WRITE for
SMB2+ protocols and using read+write for older protocols. Since the
operation may be long running, it takes a callback which gets called
periodically to indicate progress to the application and given an
opportunity to stop it.

Signed-off-by: Ross Lagerwall <rosslagerwall@gmail.com>
Reviewed-by: David Disseldorp <ddiss@samba.org>
Reviewed-by: Jeremy Allison <jra@samba.org>