Socket option NN_RCVPRIO added.

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

Signed-off-by: Martin Sustrik <sustrik@250bpm.com>
Code simplified by forwarding send/recv to sendmsg/recvmsg

Signed-off-by: Martin Sustrik <sustrik@250bpm.com>
Broken link in docs fixed

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