add NN_IPV4ONLY to nn_symbols list

Signed-off-by: Emil Renner Berthing <esmil@mailme.dk>
Signed-off-by: Martin Sustrik <sustrik@250bpm.com>
Emil Renner Berthing added to AUTHORS file

Signed-off-by: Martin Sustrik <sustrik@250bpm.com>
don't free nn_allocmsg buffer when nn_send errors

Signed-off-by: Emil Renner Berthing <esmil@mailme.dk>