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