Fix https://github.com/nplab/packetdrill/issues/131.

Improve interface handling in local mode

On *BSD platforms the original version always used the tun0 device
in local mode and never deleted it, except on NetBSD. This was
handy when using for example Wireshark for capturing trafic.
The last commit removed this possibility, since it removes the
interface. Add the --persistent_tun_dev command line option
to bring it back.

To get the behaviour of the original version, use on *BSD
platforms --tun-dev=tun0 --persistent_tun_dev.

When using local mode on *BSD, using the tun0 device was hard
coded in contrast to Linux, where the next unused tun device
was used.
This patch changes the behaviour on *BSD to use the next unused
tun device. This also requires that the device is removed when
packetdrill terminates. This was the case on Linux, but not on
*BSD.
On *BSD there is also a new command line option (--tun-dev) to
set the device name and not select an ephemerial one.

Get in sync with the nplab version

Sponsored by:	Netlix, Inc.

Sponsored by:	Netflix, Inc.

This allows to support a mixture of encapsulated and non-encapsulated
packets in a single script file.

Sponsored by:	Netflix, Inc.

Ensure sequence numbers are mapped correctly.

This adds support for SCTP/UDP and TCP/UDP including ICMP
support.

Sponsored by:	Netflix, Inc.

Improve debugging

Wrap the run time deubg option in a compile time option. (DEBUG)

Merge gnns changes to make the debug mode run time selectable.

First cut at making debug mode run time selectable

On FreeBSD IPv6 sockets have the IPV6_V6ONLY option enabled by
default. To allow the scripts to select the address family
via the --ip_version option transparently, disable the
IPV6_V6ONLY option implicilty on all sockets.
This fixes #132.

Sponsored by:	Netflix, Inc.

Fix remote mode

This fixes #114 and also the TCP remote mode.

Sync with nplab/packetdrill

Sponsored by:	Netflix, Inc.

Sponsored by:	Netflix, Inc.