Fix ofp_action_output.max_len en/de-coding; refactor related variables and atoms

The ofp_action_output.max_len field was incorrectly encoded/decoded because
it was treated the same as ofp_flow_mod.buffer_id field. Each of these fields
can be set to no_buffer but for different values. For clarity variables' names
and atoms used in encoding/decoding process are refactored.
(Motivated by FlowForwarding/LINC-Switch/#124)