netcat-openbsd: new package

The OpenBSD implementations of netcat.  The main difference from
netcat is the support for IPv6, proxies, and Unix sockets.

[Thomas: use tabs in the command applying Debian patches, add
dependency on Busybox when enabled so that we override the 'nc'
command installed by Busybox.]

Signed-off-by: Maxime Hadjinlian <maxime.hadjinlian@gmail.com>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>