selftests: Add nettest

Add nettest - a simple program with an implementation for various networking
APIs. nettest is used for tcp, udp and raw functional tests for both IPv4
and IPv6.

Point of this command versus existing utilities:
- controlled implementation of the APIs and the order in which they
  are called,
- ability to verify ingress device, local and remote addresses,
- timeout for controlled test length,
- ability to discriminate a timeout from a system call failure, and
- simplicity with test scripts.

The command returns:
  0  on success,
  1  for any system call failure, and
  2  on timeout.

Signed-off-by: David Ahern <dsahern@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>