…nnection providers

This patch introduces multiple high-level abstractions over raw low-level
sockets, which are necessary for supporting network backends other
than default legacy `TCP_DIRECT` implementation:

1. `WzConnectionProvider` - abstracts the way WZ establishes
   server-side and client-side connections. This thing effectively
   provides usable listen sockets and client connections to work with,
   hence the name.
2. `IListenSocket` - abstraction over listen sockets.
3. `IClientConnection` - abstraction over client-side sockets (and
   also server-side connections to the game clients).
4. `IConnectionPollGroup` - generalization of socket sets for polling
   multiple connections in one go.
5. `ConnectionProviderRegistry` - trivial singleton class providing
   storage for connection providers.
6. `ConnectionAddress` - opaque connection address object, aimed
   to replace direct uses of `addrinfo` and provide a bit more
   abstract way to represent connection credentials. Still looks like
   a crutch right now, but it's better than nothing, nonetheless.

The existing implementation in `netplay/netsocket.h(.cpp)` has been
moved to the `tcp` subfolder and wrapped entirely into the `tcp`
namespace.

The patch provides `TCP*`-prefixed implementations of the base
interfaces mentioned above, which are implemented in terms of the
old `netsocket` code.

There's now a `ConnectionProviderType::TCP_DIRECT` enumeration
descriptor for accessing the default connection provider.

All uses in the high-level code (`netplay.cpp`, `joiningscreen.cpp`)
are amended appropriately to use the all-new high-level abstractions
instead of old low-level tcp-specific `Socket` and `SocketSet`.

NOTE: there are still a few functions from the `tcp::` namespace
used directly in the Discord RPC integration code, but these
shouldn't pose any problem to either extract these into a more generic
abstraction layer or to be rewritten not to use these functions at all,
because they don't actually use any low-level stuff that's hard to
refactor.

Signed-off-by: Pavel Solodovnikov <[email protected]>

This way it looks much cleaner and naturally doesn't leak any
implementation details to clients. Also, now it's much
easier to provide native connection address implementations
for each backend implementation.

Signed-off-by: Pavel Solodovnikov <[email protected]>

…e `WzConnectionProvider`

Provide the default implementation for `openClientConnectionAsync()`
in `WzConnectionProvider`, which just spawns a new thread and
piggibacks on the `resolveHost()` + `openClientConnectionAny()`
combination.

The `TCPConnectionProvider` is now simpler because it can use
the default implementation from the base class.

Signed-off-by: Pavel Solodovnikov <[email protected]>