EXT-262: Instruction and action changes

Tested:
- Use "dpctl flow-mod", "dpctl stats-flow" and verify flows are
packed, unpacked and printed correctly.
- Use "dpctl table-features" and verify that table features
are packed, unpacked and printed correctly.
EXT-262: Change incorrect ofp_instruction_id to ofp_instruction_header
EXT-262: Use ofp_instruction_header in the right places.

Conflicts:

	include/openflow/openflow.h
	oflib/ofl.h
EXT-262: Change ofp_action_header to 4 bytes instead of 8.

Conflicts:

	oflib/ofl-actions-pack.c
	oflib/ofl-actions.c
	oflib/ofl-structs-pack.c
EXT-262: Change ofp_action_empty to ofp_action_generic to follow spec.
EXT-262: Fix missing case for ofl_action_generic