On Windows errno codes are translated to err strings by nanomsg rather than strerror

It seems that strerror() on Windows isn't able to convert
all error code (EISCONN in particular).

Signed-off-by: Martin Sustrik <sustrik@250bpm.com>
All error codes are reported via nn_symbol() unconditionally

Signed-off-by: Martin Sustrik <sustrik@250bpm.com>
Handle EISCONN error produced by nn_excl decently

Signed-off-by: Martin Sustrik <sustrik@250bpm.com>
tests for rejected connections added

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