cli/get: use data callback type to write out file data
cli/get: use data callback type to write out file data

In preparation for the removal of file data handling in the connection
layer, migrate cli_get to use callbacks to write out incoming data.

On data_in callback, the callee is responsible for freeing in_buf.

Signed-off-by: David Disseldorp <589a549dc9f982d9f46aeeb82a09ab6d87ccf1d8@suse.de>

ddiss/elasto,ddiss/elasto