ofp-actions: Add action bitmap abstraction.

Until now, sets of actions have been abstracted separately outside
ofp-actions, as enum ofputil_action_bitmap.  Drawing sets of actions into
ofp-actions, as done in this commit, makes for a better overall
abstraction of actions, with better consistency.

A big part of this commit is shifting from using ofp12_table_stats as if
it were an abstraction for OpenFlow table stats, toward using a new
struct ofputil_table_stats, which is what we generally do with other
OpenFlow structures and fits better with the rest of the code.

Signed-off-by: Ben Pfaff <blp@nicira.com>
Acked-by: Jarno Rajahalme <jrajahalme@nicira.com>