Implement getsockname and poll socket functions

[MmAaXx500]

Describe the project you are working on

I'm currently working on a multiplayer game which uses STUN to get around the limitations of NAT.

Describe the problem or limitation you are having in your project

getsockname: In the implementation I needed to be aware of what address the socket is bound to.
poll: I needed to set a timeout on receiving operations using an EnetSocket. So I need poll.

Describe the feature / enhancement and how it helps to overcome the problem or limitation

See above.

Describe how your proposal will work, with code, pseudo-code, mock-ups, and/or diagrams

Probably obvious. Using platform's and Godot's socket functions.
Here is the pull request: godotengine/godot#41600

If this enhancement will not be used often, can it be worked around with a few lines of script?

No it's can't be worked around because it's a core feature.

Is there a reason why this should be core and not an add-on in the asset library?

Yes! Because it's a core feature. I know no other ways to do this.