bce: Utilize hardware supplied RSS hash

The IP fragment detection needs further investigation.  Currently we
can't tell for 100% sure whether the received packet is an IP fragment
or not; we just test the hardware supplied checksum against 0xffff for
IP fragment detection.  However, it is not a big deal in the current
code base, since IP fragments reassemabling is under a token (the IP
fragments reassemabing used to be only allowed on netisr0)
ddb.4 - Mention recently added 'show tokens' command.
shlock(1) - Fix a possible overflow.