Allow user to set socket flags for server creation

[curtpm]

This will allow someone to specify NULL/nullptr for the host field and set AI_PASSIVE to allow listening to incoming requests on any connection (ie. localhost / ip4 / ip6)

[yhirose]

@curtpm, thanks for the fine contribution. I added 2 test cases for the change.
In the 2nd test, I created the client socket with 'locashost' for the server made with AI_PASSIVE, and the test passed successfully. I think it's correct, right?

[curtpm]

looks good to me!

FYI most server API's don't even support setting a host name on the "listen" function because 99% of the time you wan to listen to all connections on the system for the specified port.
ex: golang http ListenAndServe https://golang.org/pkg/net/http/#pkg-examples

Also thanks for accepting the request so fast!

[yhirose]

Thanks for the helpful information! 👍