Add the Netplay feature from 1.42n

[wilsonHodgson]

The one feature that really kept me using ZSNES was the built-in netplay.
It used it about 4 years ago to play online with a friend. Lots of fun, the windows build and the linux build worked together effortlessly.

Do you have any idea of how much effort it would involve to apply your changes to the 1.42 version? That was the last version with netplay enabled.

P.S.
I just found your contribution to ZSNES on the AUR today, and I'd like to say thank you so much for your work.
I remember trying to build it when I played years ago and I couldn't for the life of me get it working, so I had to scour websites for a sketchy linux build.

[xyproto]

I looked into this. Including the netplay-related code from 1.42n. It is possible, but not entirely straightforward.

Adding the TCP related network functions into gui/ and linux/ is relatively easy, but code also needs to be modified in gui/*.

There are also changes made after 1.42n that conflicts with the latest code.

I added the netplay branch that includes the Netplay-enabled source code in the netplay folder.

For now, I'll leave this as a task for another day. I'll leave this ticket open in case someone feels inspired to look into this.