LIB: Add options into cmake about demo and documentation.
LIB: Fix "unreachable code" warnings.
LIB: Interrupt IO in threads.
DOC: Describe little Socket::Set class, PREGAME state. Fix some typos.
LIB: Make Socket class properly moveable. Add Socket::Set class.
DOC: Add "tcp-ipv6" option.
LIB: Move some socket code to Socket::Set class. Add IPv6 support.