ofp-actions: Split ofpacts_check__() into many functions.

ofp-actions: Split ofpacts_check__() into many functions.

ofpacts_check__() was a huge switch statement with special cases for many
different kinds of actions.  This made it unwieldy and put the special
cases far away from the rest of the code related to a given action.  This
commit refactors the code to avoid the problem.

Signed-off-by: Ben Pfaff <blp@ovn.org>
Tested-by: Yifeng Sun <pkusunyifeng@gmail.com>
Reviewed-by: Yifeng Sun <pkusunyifeng@gmail.com>