NN_SNDFD/NN_RCVFD/NN_ERRFD are created immediately on socket creation

Signed-off-by: Martin Sustrik <sustrik@250bpm.com>
nn_efd_init can return EMFILE

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

Termination of the library is now signaled by both POLLIN and POLLOUT

Signed-off-by: Martin Sustrik <sustrik@250bpm.com>
nn_term(3) man page updated to reflect behavior of NN_SNDFD and NN_RCVFD

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