nn_send assert when IPv4only option was disabled, and try to connect with IPv4 address

[clearday4]

I'm testing the PUSH/PULL socket using TCP transport.
If IPv4only option was disabled, and try to connect with IPv4 address,
the destination address resolved AF_INET, but local address which the socket would bind resolved as AF_INET6.
so nn_usock_start makes the socket AF_INET.
after that nn_ctcp_start_connecting() try to bind the local IPv6 address, it was failed.
the local address lookup code is not working properly.
nn_iface_any determines the bind address only with ipv4only option.

after that, nanomsg try to reconnect and takes a same routine again.
at that time usock->state is not idle, it is NN_USOCK_STATE_STARTING,
so nn_usock_start is assert.

please consider the following three points

1. nn_iface_any should not determine the address with ipv4only option
2. nn_usock_start should consider connection retry case.
3. if ipv4only option is disabled, make the socket as AF_INET6 forced.

[clearday4]

I move this issue to main branch