o Clean up interface between ip_fw_chk() and its callers:

• ip_fw_chk() returns action as function return value. Field retval is removed from args structure. Action is not flag any more. It is one of integer constants.
• Any action-specific cookies are returned either in new "cookie" field in args structure (dummynet, future netgraph glue), or in mbuf tag attached to packet (divert, tee, some future action).

o Convert parsing of return value from ip_fw_chk() in ipfw_check_{in,out}()

to a switch structure, so that the functions are more readable, and a future
actions can be added with less modifications.

Approved by: andre
MFC after: 2 months