Update pkt-gen to optionally use randomised source/destination
IPv4 addresses/ports.

When doing traffic testing of actual code that /does/ things to the
packet (rather than say, 'bridge.c'), it's typically a good idea to
use a variety of cache-busting and flow-tracking-busting packet
spreads. The pkt-gen method of testing an IP range was to walk
it linearly - which is fine, but not useful enough.

This can be used to completely randomize the source/destination
addresses (eg to test out flow-tracking-busting) and to keep the
destination fixed whilst randomising the source (eg to test out
what a DDoS may look like.)

Tested:

• Intel ixgbe 10G (82599) netmap

Differential Revision: https://reviews.freebsd.org/D2309
MFC after: 2 weeks
Sponsored by: Norse Corp, Inc.