Allow netcat to be a tun tunnel broker

[thj]

Summary

Add tests with this new functionality to test that tun devices can pass packets.

You need to figure both sides with netcat, one side should listen:

1. ifconfig tun create

tun0

1. nc -u -l --tun /dev/tun0 192.168.100.12 2600 &
2. ifconfig tun0 inet 10.4.100.2/24 10.4.100.3 up

1. ifconfig tun create

tun0

1. nc -u --tun /dev/tun0 192.168.100.12 2600 &
2. ifconfig tun0 inet 10.4.100.2/24 10.4.100.2 up

This is very naiive and probably quite fragile, but it is enough to enable tun
tests using just netcat.

Add usage and help strings

Add tun tunnelling test using nc

[kevans]

Just some minor simplifications, I think we can just use tun_fd being a valid fd as an indicator that we should be tunneling instead of tracking that in separate state.

contrib/netcat/netcat.c

150

Normally we just use CHAR_MAX to make it clear

164

IMO, let's init tundev to NULL here

337

tundev could be a const char * and you can avoid having to make a copy of optarg here, since we don't need to mutate it later.

339

Drop FreeBSD_tun completely, the below conditional can use tundev != NULL

586

tun_fd >= 0, since we initialize it to -1

864

Let's use tun_fd >= 0 for these ones, as well

[thj]

Address Kyles review comments

[bcr]

OK for the man page part of the change.

[kevans]

Seems to LGTM; thanks!