sp_ prefixes changed to nn_ prefixes

Signed-off-by: Martin Sustrik <sustrik@250bpm.com>
Warning about file descriptors added to nn_socket man page

Signed-off-by: Martin Sustrik <sustrik@250bpm.com>
File descriptor warning in nn_socket man page moved to RETURN VALUE section

Signed-off-by: Martin Sustrik <sustrik@250bpm.com>
Man pages adjusted to reflect changes to nn_sendmsg and nn_recvmsg

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