from_pipe: remove unused port in packet_out

The port optional port in a OFPT_PACKET_OUT is only used when the action
list contains an action that would send the packet to the TABLE
pseudoport. As of now, Frenetic does not allow this.
from_pipe: move result type from NetKAT_Types to Async_NetKAT
from_pipe: Document Async_NetKAT