[Feature Request] Configurable network binding

When configuring the network control interface, you can set the port and that's about it. Per the docs:

network_cmd_enable = "true"
network_cmd_port = "55355"

I was hoping I would be able to change the bind address so I can secure my deployment a bit, but looking in the code, RetroArch calls socket_init with hardcoded server = NULL. It appears that the rest of the code treats this scenario as binding to 0.0.0.0.

I'd love to be able to set the bind address in the config -- something like:

network_cmd_enable = "true"
network_cmd_listen = "127.0.0.1"
network_cmd_port = "55355"

(Obviously, we'd leave the default bind address to 0.0.0.0 so as not to break existing deployments.)

This would make it so that anyone trying to automate RetroArch could do so without opening their systems to remote unauthenticated ACE (albeit restricted to the emulation core). My particular use case is a kiosk, but I could also see it being useful on cabinets, console-like systems, and less restrictive kiosks.

Start in command.c at the network control interface setup where socket_init is called with server set to NULL, then trace how network_cmd_enable and network_cmd_port are read from configuration. Add support for a network_cmd_listen setting while preserving the existing default bind address, and verify that the interface still starts with the documented settings.