Feature request: Fix crash by setting joinable property of sessions

[gynt]

This is a feature request

Currently, games can crash (e.g., Stronghold Crusader), if users join a lobby of an already active game since the original game code usually doesn't account for such a situation. Both DirectPlay (NewPlayersDisabled flag) and Steam support locking of lobbies.

An implementation would mean some conversion between Steam and DirectPlay lobbies, but should be doable hopefully. Note that Steam will not list lobbies that are not joinable, while DirectPlay a EnumSessions request can technically ask for non-joinable lobbies.

[gynt]

Implementation:
This code needs to update the lobby data based on the incoming session decription. In our case, check for the NewPlayersDisabled flag if it is set.

RedirectPlay/RedirectPlay/ServiceProviders/Steamworks/SteamPlayProvider.cpp

Lines 501 to 504 in a4e4092

	HRESULT CSteamPlayProvider::SetSessionDesc(LPDPSESSIONDESC2 description, DWORD flags)
	{
	return DP_OK;
	}

with a call to:
https://partner.steamgames.com/doc/api/ISteamMatchmaking#SetLobbyJoinable