With an upcoming patch the output target can be different to `stdout`.
Therefore, the term `ZG_ACTION_STDOUT` is no longer generic enough. In addition,
rename `do_write` to `do_copy` to match the enum name.
Reviewed-by: Alexander Egorenkov <egorenar@linux.ibm.com>
Signed-off-by: Marc Hartmayer <mhartmay@linux.ibm.com>
Signed-off-by: Jan Höppner <hoeppner@linux.ibm.com>