ahaggar: convert "struct output_buffer" to "output_buf" so we can compile with g++

Signed-off-by: Djalal Harouni <tixxdz@opendz.org>
ahaggar: more output_buffer to output_buf conversion

Signed-off-by: Djalal Harouni <tixxdz@opendz.org>
ahaggar: complete the output_buf conversion

Signed-off-by: Djalal Harouni <tixxdz@opendz.org>