Implement flowid calculation for outbound connections to balance

Implement flowid calculation for outbound connections to balance
 connections over multiple paths.

Multipath routing relies on mbuf flowid data for both transit
 and outbound traffic. Current code fills mbuf flowid from inp_flowid
 for connection-oriented sockets. However, inp_flowid is currently
 not calculated for outbound connections.

This change creates simple hashing functions and starts calculating hashes
 for TCP,UDP/UDP-Lite and raw IP if multipath routes are present in the
 system.

Reviewed by:	glebius (previous version),ae
Differential Revision:	https://reviews.freebsd.org/D26523