Overwrite sndfd and rcvfd objects by 0xcd when not used

This  is useful for debugging purposes.

Signed-off-by: Martin Sustrik <sustrik@250bpm.com>
NN_DOMAIN and NN_PROTOCOL socket options added

Signed-off-by: Martin Sustrik <sustrik@250bpm.com>
Full nn_device() implementation

Signed-off-by: Martin Sustrik <sustrik@250bpm.com>
nn_device(3) man page improved

Signed-off-by: Martin Sustrik <sustrik@250bpm.com>