Implement siphash routines for initial TCP sequence numbers (12 bytes
input for IPv4, 36 bytes input for IPv6), and while at it, also
functions we'll use later on for hash table indices and TCP timestamp
offsets (with 8, 20, 32 bytes of input).
Use these to set the initial sequence number, according to RFC 6528,
for connections originating either from the tap device or from
sockets.
Signed-off-by: Stefano Brivio <sbrivio@redhat.com>
