Fixes for old interfaces

[NicknineTheEagle]

This adds a number of fixes for old Steam interfaces:

• A game can call ISteamClient::ConnectToGlobalUser multiple times, this is valid usage of API. Under Goldberg, this currently causes double init, so I have fixed that.
• In SteamClient004, HSteamUser is actually a unique user ref index, not a user index, so the current behavior causes use-after-free issues. I've reworked the legacy user create/release functions to emulate that behavior.
• I've made several RE mistakes when implementing ICMCallback for pre-SteamClient004 interfaces, this PR fixes them.